radius_next.cpp

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/* This file is part of Cloudy and is copyright (C)1978-2022 by Gary J. Ferland and
 * others.  For conditions of distribution and use see copyright notice in license.txt */
/*radius_next use adaptive logic to find next zone thickness */
/*ContRate called by radius_next to find energy of maximum continuum-gas interaction */
/*GrainRateDr called by radius_next to find grain heating rate dr */
#include "cddefines.h"
#include "iso.h"
#include "geometry.h"
#include "h2.h"
#include "hyperfine.h"
#include "opacity.h"
#include "dense.h"
#include "heavy.h"
#include "grainvar.h"
#include "elementnames.h"
#include "dynamics.h"
#include "thermal.h"
#include "hmi.h"
#include "coolheavy.h"
#include "timesc.h"
#include "stopcalc.h"
#include "colden.h"
#include "phycon.h"
#include "rt.h"
#include "trace.h"
#include "wind.h"
#include "save.h"
#include "pressure.h"
#include "iterations.h"
#include "struc.h"
#include "radius.h"
#include "dark_matter.h"
#include "mole.h"
#include "rfield.h"
#include "freebound.h"
#include "lines_service.h"
#include "cosmology.h"

/*ContRate called by radius_next to find energy of maximum continuum-gas interaction */
STATIC void ContRate(double *freqm, 
                     double *opacm);

/*GrainRateDr called by radius_next to find grain heating rate dr */
STATIC void GrainRateDr(double *grfreqm, 
                        double *gropacm);

class drChoiceItem
{
        string m_str;
        bool *m_flag;
public:
        drChoiceItem(const string& str, bool *flag) : m_str(str), m_flag(flag) {}
        void select() const
        {
                if (m_flag)
                        *m_flag = true;
        }
        const char* c_str() const
        {
                return m_str.c_str();
        }
};

class drList
{
        multimap<double,drChoiceItem> m_map;
public:
        typedef multimap<double,drChoiceItem>::const_iterator const_iterator;
        void insert(double val, const string& str, bool* flag = NULL)
        {
                if (flag != NULL)
                        *flag = false;
                m_map.insert(pair<const double,drChoiceItem>(val,drChoiceItem(str,flag)));
        }
        void clear()
                {
                m_map.clear();
        }
        const_iterator begin() const
        {
                return m_map.begin();
        }
        const_iterator end() const
        {
                return m_map.end();
        }
        size_t size() const
        {
                return m_map.size();
        }
};


/*radius_next use adaptive logic to find next zone thickness 
 * return 0 if ok, 1 for abort */
int radius_next(void)
{
        static double OHIIoHI, 
          OldHeat = 0., 
          OldTe = 0.,
          OlddTdStep = 0.,
          OldWindVelocity = 0.,
          Old_H2_heat_cool;

        const double BigRadius = 1e30;

        DEBUG_ENTRY( "radius_next()" );

        /*--------------------------------------------------------------
         *
         * this sub determines the thickness of the next zone
         * if is called one time for each zone
         *
         *-------------------------------------------------------------- */

        if( trace.lgTrace )
                fprintf( ioQQQ, "   radius_next called\n" );

        /* >>chng 03 sep 21 - decide whether this is the first call */
        bool lgFirstCall = ( nzone == 0 );

        drList drChoice;

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
         * check on change in hydrogen ionization */
        if( !lgFirstCall && OHIIoHI > 1e-15 &&
            (dense.xIonDense[ipHYDROGEN][1] > 0. && dense.xIonDense[ipHYDROGEN][0] > 0.) )
        {
                double atomic_frac = dense.xIonDense[ipHYDROGEN][1]/dense.xIonDense[ipHYDROGEN][0];
                double drHion;
                /* ratio of current HII/HI to old value - < 1 when becoming more neutral */
                /* this is now change in atomic fraction */
                double x = fabs(1. - atomic_frac/OHIIoHI);
                if( atomic_frac > 0.05 && atomic_frac < 0.9 )
                {
                        /* >>chng 96 feb 23 from 0.7 to 0.3 because punched through H i-front
                         * >>chng 97 may 5, from 0.3 to 0.2 since punched through H i-front */
                        /* >>chng 02 jun 05 from 0.2 to 0.05 poorly resolved i-front, also added two-branch logic*/
                        drHion = radius.drad*MAX2( 0.2 , 0.05/MAX2(1e-10,x) );
                }
                else
                {
                        /* >>chng 96 feb 23 from 0.7 to 0.3 because punched through H i-front
                         * >>chng 97 may 5, from 0.3 to 0.2 since punched through H i-front */
                        drHion = radius.drad*MAX2( 0.2 , 0.2/MAX2(1e-10,x) );
                }
                double SaveOHIIoHI = OHIIoHI;
                OHIIoHI = dense.xIonDense[ipHYDROGEN][1]/dense.xIonDense[ipHYDROGEN][0];
                ostringstream oss;
                oss << "change in H ionization old=" << scientific << setprecision(3);
                oss << SaveOHIIoHI << " new=" << OHIIoHI;
                drChoice.insert( drHion, oss.str() );
        }
        else
        {
                if( dense.xIonDense[ipHYDROGEN][1] > 0. && dense.xIonDense[ipHYDROGEN][0] > 0. )
                        OHIIoHI = dense.xIonDense[ipHYDROGEN][1]/dense.xIonDense[ipHYDROGEN][0];
                else
                        OHIIoHI = 0.;
        }

        if( rt.dTauMase < -0.01 )
        {
                /* maser so powerful, must limit inc in tay
                 * >>chng 96 aug 08, from 0.3 to 0.1 due to large maser crash */
                double drHMase = radius.drad*MAX2(0.1,-0.2/rt.dTauMase);
                ostringstream oss;
                //
                // NB NB -- DON'T ALTER THIS STRING, setting rt.lgMaserSetDR below keys from this!
                // No longer true:
                // >>chng 13 oct 01 rjrw take logical pointer argument for
                // >>optional flag to be set if selected
                //
                oss << "H maser dTauMase=" << scientific << setprecision(2) << rt.dTauMase << " ";
                oss << rt.mas_species << " " << rt.mas_ion << " " << rt.mas_hi << " " << rt.mas_lo;
                drChoice.insert( drHMase, oss.str(), &rt.lgMaserSetDR );
        }

        /* check change in relative ionization - this is to insure a gradual approach to
         * ionization fronts - were dr allowed to remain constant we would crash through
         * a front in a single zone */
        double change_heavy_frac_big = -1.;
        double frac_change_heavy_frac_big = -1.;
        const realnum CHANGE_ION_HEAV = 0.2f;
        const realnum CHANGE_ION_HHE = 0.15f;
        if( nzone > 4 )
        {
                long ichange_heavy_nelem = -1L;
                long ichange_heavy_ion = -1L;
                double dr_change_heavy = BigRadius;
                for( int nelem=ipHYDROGEN; nelem<LIMELM; ++nelem )
                {
                        if( dense.lgElmtOn[nelem] )
                        {
                                realnum change;
                                /* will only track ions with fractional abundances greater than this
                                 * He and C are special cases because of special roles in PDRs */
                                realnum frac_limit;
                                if( nelem<=ipHELIUM || nelem==ipCARBON )
                                {
                                        frac_limit = 1e-4f;
                                        change = CHANGE_ION_HHE;
                                }
                                else
                                {
                                        /* struc.dr_ionfrac_limit set in init_coreload, is limit to fractional
                                         * abundances of ions that are used in prt_comment to indicate oscillations
                                         * or rapid change in ionization */
                                        frac_limit = struc.dr_ionfrac_limit/2.f;
                                        change = CHANGE_ION_HEAV;
                                }

                                for( int ion=0; ion<=nelem+1; ++ion )
                                {
                                        realnum abundnew = dense.xIonDense[nelem][ion]/SDIV( dense.gas_phase[nelem]);
                                        realnum abundold = struc.xIonDense[nzone-3][nelem][ion]/SDIV( struc.gas_phase[nzone-3][nelem]);
                                        if( abundnew > frac_limit && abundold > frac_limit )
                                        {
                                                /* this test is not done when [nzone-n] <0 */
                                                realnum abundolder = struc.xIonDense[nzone-4][nelem][ion]/SDIV( struc.gas_phase[nzone-4][nelem]);
                                                realnum abundoldest = struc.xIonDense[nzone-5][nelem][ion]/SDIV( struc.gas_phase[nzone-5][nelem]);
                                                /* this is fractional change, use min of old and new abundances, to pick up
                                                 * rapid changing Ca+ sooner */
                                                double change_heavy_frac = fabs(abundnew-abundold)/MIN2(abundold,abundnew);
                                                /* want fractional change to be less than this factor */
                                                if( (change_heavy_frac > change) && (change_heavy_frac > change_heavy_frac_big) &&
                                                        /* >>chng 03 dec 07, add test that abund is not oscillating */
                                                        /* also test that abundance is increasing - we are headed into a front */
                                                        ((abundnew-abundold)>0.)   && 
                                                        ((abundold-abundolder)>0.) && 
                                                        ((abundolder-abundoldest)>0.) )
                                                {
                                                        ichange_heavy_nelem = nelem;
                                                        ichange_heavy_ion = ion;
                                                        change_heavy_frac_big = change_heavy_frac;
                                                        frac_change_heavy_frac_big = abundnew;
                                                        /* factor in max has been adjusted to prevent code from running
                                                         * through various ionization fronts.
                                                         * >>chng 03 dec 06, from min of 0.5 to min of 0.25, crash into He i-front
                                                         * in hizqso.in */
                                                        /* >>chng 04 mar 03, min had become 0.1, forced oscillations in nova.in
                                                         * in silicon, zoning changed greatly, causing change in diffuse lin
                                                         * pumping.  put back to 0.25 */
                                                        dr_change_heavy = radius.drad * MAX2(0.25, change / change_heavy_frac );
                                                }
                                        }
                                }
                        }
                }
                /* set to -1 before loop over ions, if no significant changes in ionization occurred
                 * then may still be -1
                 */
                ostringstream oss;
                if(ichange_heavy_nelem>=0)
                {
                        oss << "change in ionization, element " << elementnames.chElementName[ichange_heavy_nelem];
                        oss << " ion (C scale) " << ichange_heavy_ion << " rel change " << scientific << setprecision(2);
                        oss << change_heavy_frac_big << " ion frac " << frac_change_heavy_frac_big;
                }
                else
                {
                        oss << "none";
                        dr_change_heavy = BigRadius;
                }
                drChoice.insert( dr_change_heavy, oss.str() );
        }

        /* if Leiden hacks are on then use increase in dust extinction as control 
         * >>chng 05 aug 13, add this */
        if( mole_global.lgLeidenHack )
        {
                /* Draine field is only defined over narrow range in FUV - must not let change
                 * in extinction become too large - 
                 * prefactor is change in optical depth */
                double drLeiden_hack = MAX2( 0.02 , 0.05*rfield.extin_mag_V_point ) /
                        SDIV( geometry.FillFac * rfield.opac_mag_V_point );
                drChoice.insert( drLeiden_hack, "Leiden hack" );
        }

        // limit to dust optical depth per zone
        static double extin_mag_V_point_old = -1.;
        if( nzone>1 )
        {
                 const double extin_mag_V_limit = 20.+0.01*extin_mag_V_point_old;
                 double dExtin = (rfield.extin_mag_V_point - extin_mag_V_point_old)/extin_mag_V_limit;
                 if( dExtin > SMALLFLOAT )
                 {
                          double drExtintion = radius.drad / dExtin;
                                ostringstream oss;
                                oss << "change in V mag extinction; current=" << scientific << setprecision(3) <<
                                         rfield.extin_mag_V_point;
                                oss << fixed << " delta=" << dExtin;
                                drChoice.insert( drExtintion, oss.str() );
                 }

        }
        extin_mag_V_point_old = rfield.extin_mag_V_point;

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* check how heating is changing */
        if( nzone > 1 && !thermal.lgTemperatureConstant &&
            /* if density fluctuations in place then override change in heat for dr set */
            !( strcmp(dense.chDenseLaw,"SINE") == 0 && dense.lgDenFlucOn ) )
        {
                double dHdStep = safe_div(fabs(thermal.htot-OldHeat),thermal.htot,0.0);
                if( dHdStep > 0. )
                {
                        double drdHdStep;
                        if( dense.gas_phase[ipHYDROGEN] >= 1e13 )
                        {
                                drdHdStep = radius.drad*MAX2(0.8,0.075/dHdStep);
                        }
                        else if( dense.gas_phase[ipHYDROGEN] >= 1e11 )
                        {
                                drdHdStep = radius.drad*MAX2(0.8,0.100/dHdStep);
                        }
                        else
                        {
                                /* changed from .15 to .12 for outer edge of coolhii too steep dT
                                 * changed to .10 for symmetry, big change in some rates, 95nov14
                                 * changed from .10 to .125 since parispn seemed to waste zones
                                 * >>chng 96 may 21, from .125 to .15 since pn's still waste zones */
                                drdHdStep = radius.drad*MAX2(0.8,0.15/dHdStep);
                        }
                        ostringstream oss;
                        oss << "change in heating; current=" << scientific << setprecision(3) << thermal.htot;
                        oss << fixed << " delta=" << dHdStep;
                        drChoice.insert( drdHdStep, oss.str() );
                }
        }
        OldHeat = thermal.htot;

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* pressure due to incident continuum if in eos */
        if( strcmp(dense.chDenseLaw,"CPRE") == 0 && pressure.lgContRadPresOn )
        {
                if( nzone > 2 && pressure.pinzon > 0. )
                {
                        /* pinzon is pressrue from acceleration onto previos zone
                         * want this less than some fraction of total pressure */
                        /* >>chng 06 feb 01, change from init pres to current total pressure
                         * in const press high U ulirgs current pressure may be quite larger
                         * than init pressure due to continuum absorption */
                        double drConPres = 0.05*pressure.PresTotlCurr/
                                (wind.AccelTotalOutward*dense.xMassDensity*geometry.FillFac);
                        drChoice.insert( drConPres, "change in con accel" );
                }
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* gravitational pressure */
        if( strcmp(dense.chDenseLaw,"CPRE") == 0 && (dark.lgNFW_Set || pressure.gravity_symmetry>=0) )
        {
                if( fabs( pressure.RhoGravity ) > 0. )
                {
                        double drGravPres = 0.05 * pressure.PresTotlCurr * radius.drad / fabs( pressure.RhoGravity );
                        ASSERT( drGravPres > 0. );
                        drChoice.insert( drGravPres, "change in gravitational pressure" );
                }
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* check how temperature is changing */
        double dTdStep = FLT_MAX;
        if( nzone > 1 )
        {
                /* change in temperature; current=      */
                dTdStep = (phycon.te-OldTe)/phycon.te;
                /* >>chng 02 dec 08, desired change in temperature per zone must not
                 * be smaller than allower error in temperature.  For now use relative error
                 * in heating - - cooling balance.  Better would be to also use c-h deriv wrt t
                 * to get slope */
                /* >>chng 02 dec 11 rjrw change back to 0.03 -- improve dynamics.dRad criterion instead */
                double x = 0.03;
                /* >>chng 02 dec 07, do not do this if there is mild te jitter, 
                 * so that dT is changing sign - this happens
                 * in ism.ini, very stable temperature with slight noise up and down */
                if( dTdStep*OlddTdStep > 0. )
                {
                        /* >>chng 96 may 30, variable depending on temp
                         * >>chng 96 may 31, allow 0.7 smaller, was 0.8
                         * >>chng 97 may 05, from 0.7 to 0.5 stop from punching through thermal front
                         * >>chng 04 mar 30, from 0.7 to 0.5 stop from punching through thermal front,
                         * for some reason factor was 0.7, not 0.5 as per previous change */
                        double absdt = fabs(dTdStep);
                        double drdTdStep = radius.drad*MAX2(0.5,x/absdt);
                        ostringstream oss;
                        oss << "change in temperature; current=" << scientific << setprecision(3) << phycon.te;
                        oss << fixed << " dT/T=" << dTdStep;
                        drChoice.insert( drdTdStep, oss.str() );
                }
        }
        OlddTdStep = dTdStep;
        OldTe = phycon.te;

        /* >>chng 02 oct 06, only check on opacity - interaction if not
         * constant temperature - there were constant temperature models that
         * extended to infinity but hung with last few photons and this test.
         * better to ignore this test which is really for thermal models */
        /* >>chng 06 mar 20, do not call if recombination logic in place */
        if( !thermal.lgTemperatureConstant && !dynamics.lgRecom )
        {
                double freqm = 0., opacm = 0.;
                /* find freq where opacity largest and interaction rate is fastest */
                ContRate(&freqm,&opacm);

                /* use optical depth at max interaction energy
                * >>chng 96 jun 06 was drChange=0.15 changed to 0.3 for high Z models
                * taking too many zones
                * drInter = drChange / MAX(1e-30,opacm*FillFac) */

                double drInter = 0.3/MAX2(1e-30,opacm*geometry.FillFac*geometry.DirectionalCosin);
                ostringstream oss;
                oss << "cont inter nu=" << scientific << setprecision(2) << freqm << " opac=" << opacm;
                drChoice.insert( drInter, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* check whether change in wind velocity constrains DRAD 
         * WJH 22 May 2004: disable when we are near the sonic point since
         * the velocity may be jumping all over the place but we just want
         * to push through it as quickly as we can */
        if( !wind.lgStatic() && !pressure.lgSonicPoint && !pressure.lgStrongDLimbo )
        {
                double v = fabs(wind.windv);
                /* this is relative change in velocity */
                double dVelRelative = fabs(wind.windv-OldWindVelocity)/
                        MAX2(v,0.1*timesc.sound_speed_isothermal);

                const double WIND_CHNG_VELOCITY_RELATIVE = 0.01;
                /*fprintf(ioQQQ,"DEBUG rad %.3f vel %.2e dVelRelative %.2e", 
                        log10(radius.Radius) , wind.windv ,  dVelRelative );*/

                /* do not use this on first zone since do not have old velocity */
                double winddr;
                if( dVelRelative > WIND_CHNG_VELOCITY_RELATIVE  && nzone>1 )
                {
                        /* factor less than one if change larger than WIND_CHNG_VELOCITY_RELATIVE */
                        double factor = WIND_CHNG_VELOCITY_RELATIVE / dVelRelative;
                        /*fprintf(ioQQQ," DEBUG factor %.2e", factor );*/
                        factor = MAX2(0.8 , factor );
                        winddr = radius.drad * factor;
                }
                else
                {
                        winddr = BigRadius;
                }

                /* set dr from advective term */
                if( dynamics.lgAdvection && iteration > dynamics.n_initial_relax)
                {
                        winddr = MIN2( winddr , dynamics.dRad );
                        /*>>chng 04 oct 06, set dVelRelative to dynamics.dRad since dVelRelative is printed as part
                         * of reason for choosing this criteria, want to reflect valid reason. */
                        dVelRelative = dynamics.dRad;
                }
                ostringstream oss;
                oss << "Wind, dVelRelative=" << scientific << setprecision(3);
                oss << dVelRelative << " windv=" << wind.windv;
                drChoice.insert( winddr, oss.str() );
        }
        /* remember old velocity */
        OldWindVelocity = wind.windv;

        const double DNGLOB = 0.10;

        /* inside out globule */
        if( strcmp(dense.chDenseLaw,"GLOB") == 0 )
        {
                if( radius.glbdst < 0. )
                {
                        fprintf( ioQQQ, " Globule distance is negative, internal overflow has occured, sorry.\n" );
                        fprintf( ioQQQ, " This is routine radius_next, GLBDST is%10.2e\n", 
                          radius.glbdst );
                        cdEXIT(EXIT_FAILURE);
                }
                double factor = radius.glbden*pow(radius.glbrad/radius.glbdst,radius.glbpow);
                double fac2 = radius.glbden*pow(radius.glbrad/(radius.glbdst - (realnum)radius.drad),radius.glbpow);
                /* DNGLOB is relative change in density allowed this zone, 0.10 */
                double GlobDr = ( fac2/factor > 1. + DNGLOB ) ? radius.drad*DNGLOB/(fac2/factor - 1.) : BigRadius;
                GlobDr = MIN2(GlobDr,radius.glbdst/20.);
                ostringstream oss;
                oss << "GLOB law, HDEN=" << scientific << setprecision(2) << dense.gas_phase[ipHYDROGEN];
                drChoice.insert( GlobDr, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        double hdnew = 0.;
        if( strncmp( dense.chDenseLaw , "DLW" , 3) == 0 )
        {
                /* one of the special density laws, first get density at possible next dr */
                if( strcmp(dense.chDenseLaw,"DLW1") == 0 )
                {
                        hdnew = dense_fabden(radius.Radius+radius.drad,radius.depth+
                          radius.drad);
                }
                else if( strcmp(dense.chDenseLaw,"DLW2") == 0 )
                {
                        hdnew = dense.DLW.tabval(radius.Radius+radius.drad,radius.depth+
                          radius.drad);
                }
                else if( strcmp(dense.chDenseLaw,"DLW3") == 0 )
                {
                        hdnew = dense_parametric_wind(radius.Radius+radius.drad);
                }
                else
                {
                        fprintf( ioQQQ, " dlw insanity in radius_next\n" );
                        cdEXIT(EXIT_FAILURE);
                }
                double drTab = fabs(hdnew-dense.gas_phase[ipHYDROGEN])/MAX2(hdnew,dense.gas_phase[ipHYDROGEN]);
                drTab = radius.drad*MAX2(0.2,0.10/MAX2(0.01,drTab));
                ostringstream oss;
                oss << "spec den law, new old den " << scientific << setprecision(2);
                oss << hdnew << " " << dense.gas_phase[ipHYDROGEN];
                drChoice.insert( drTab, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* special density law */
        if( strcmp(dense.chDenseLaw,"DLW1") == 0 )
        {
                double dnew = fabs(dense_fabden(radius.Radius+radius.drad,radius.depth+
                  radius.drad)/dense.gas_phase[ipHYDROGEN]-1.);
                /* DNGLOB is relative change in density allowed this zone, 0.10 */
                double SpecDr;
                if( dnew == 0. )
                {
                        SpecDr = radius.drad*3.;
                }
                else if( dnew/DNGLOB > 1.0 )
                {
                        SpecDr = radius.drad/(dnew/DNGLOB);
                }
                else
                {
                        SpecDr = BigRadius;
                }
                ostringstream oss;
                oss << "special law, HDEN=" << scientific << setprecision(2) << dense.gas_phase[ipHYDROGEN];
                drChoice.insert( SpecDr, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* check grain line heating dominates
         * this is important in PDR and HII region calculations
         * >>chng 97 jul 03, added following check */
        if( safe_div(thermal.heating(0,13),thermal.htot,0.0) > 0.2 )
        {
                double grfreqm = 0., gropacm = 0.;
                /* >>chng 01 jan 03, following returns 0 when NO light at all,
                 * had failed with botched monitor */
                GrainRateDr(&grfreqm,&gropacm);
                double DrGrainHeat = 1.0/MAX2(1e-30,gropacm*geometry.FillFac*geometry.DirectionalCosin);
                ostringstream oss;
                oss << "grain heating nu=" << scientific << setprecision(2) << grfreqm << " opac=" << gropacm;
                drChoice.insert( DrGrainHeat, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* check if line heating dominates
         * this is important in high metallicity models */
        if( safe_div(thermal.heating(0,22),thermal.htot,0.0) > 0.2 )
        {
                long level = -1;
                TransitionProxy t = FndLineHt(&level);

                if( t.Coll().heat()/thermal.htot > 0.1 )
                {
                        ASSERT( t.associated() );

                        double TauInwd = t.Emis().TauIn();
                        double DopplerWidth = t.Emis().dampXvel()/t.Emis().damp();      
                        double TauDTau = t.Emis().VoigtLineCen()*t.Emis().PopOpc()*t.Emis().opacity()/DopplerWidth;

                        fixit("all other line stacks need to be included here.");
                        // can we just sweep over line stack?  Is that ready yet?

                        /* in following logic cannot use usual inverse opacity,
                         * since line heating competes with escape probability,
                         * so is effective at surprising optical depths */
                        double drLineHeat = ( TauDTau > 0. ) ? MAX2(1.,TauInwd)*0.4/TauDTau : BigRadius;
                        ostringstream oss;
                        oss << "level " << level << " line heating, " << chLineLbl(t) << " TauIn " << scientific;
                        oss << setprecision(2) << TauInwd << " " << t.Emis().pump() << " " << t.Emis().Pesc();
                        drChoice.insert( drLineHeat, oss.str() );
                }
        }

        /* do not let change in cooling/heating due to H2 become too large */
        if( lgFirstCall )
                Old_H2_heat_cool = 0.;
        else if( !thermal.lgTemperatureConstant )
        {
                /* this is case where temperature has not been set */
                /* compare total heating - cooling due to h2 with total due to everything */
                double H2_heat_cool = safe_div((fabs(hmi.HeatH2Dexc_used)+fabs(hmi.HeatH2Dish_used)), thermal.htot,0.0);
                // Leiden hack PDR sims can have H2 heating set by sims equation, does not change
                double dH2_heat_cool = fabs( H2_heat_cool - Old_H2_heat_cool );
                if( H2_heat_cool > 0.1 && Old_H2_heat_cool>0. && dH2_heat_cool>SMALLFLOAT )
                {
                        /* >>chng 05 oct 27, had been taking 20% of original radius - this caused zoning
                         * to become very fine and may not have been needed - change from 0.2 to 0.5 */
                        double drH2_heat_cool = radius.drad*MAX2(0.5,0.05/dH2_heat_cool);
                        ostringstream oss;
                        oss << "change in H2 heating/cooling, d(c,h)/H " << scientific << setprecision(2);
                        oss << dH2_heat_cool;
                        drChoice.insert( drH2_heat_cool, oss.str() );
                }
        }
        Old_H2_heat_cool = safe_div((fabs(hmi.HeatH2Dexc_used)+fabs(hmi.HeatH2Dish_used)) , thermal.htot, 0.0);

        /* >>chng 03 mar 04, add this logic */
        /* do not let change in H2 and heavy element molecular abundances become too large 
         * "change in heav ele mole abundance, d(mole)/elem" */
        if( nzone >= 4 )
        {
                /* first do H2 abundance */
                /* >>chng 04 dec 15, do not use special logic when large h2 is turned on */
                double Old_H2_abund = struc.H2_abund[nzone-3];
                /* >>chng 03 jun 16, limit from 0.01 to 0.001, some models fell over H2 front due to
                 * large zoning, when large H2 was just this caused oscillations in solomon process */
                /* >>chng 03 nov 22, from > 0.001 to > 3e-4, models that start almost in H2 have
                 * rapid increase in H2 at shallow depths, start sensing this sooner */
                /* >>chng 03 dec 10, from 3e-4 to 1e-4, to get smaller chagned in leiden1.in test */
                /* radius_next keys from change in H2 abundance, d(H2)/H */
                /* >>chng 04 mar 11, start sensing H2 earlier since otherwise step size
                 * needs to become way too small tooo quickly.  limit changed from 1e-4 to 1e-6 */
                /* >>chng 04 jun 29, fromo > 1e-6 to >1e-8, some pdr's had too large a change in H2 */

                if( 2.*hmi.H2_total/dense.gas_phase[ipHYDROGEN] > 1e-8 )
                {
                        double fac = 0.2;
                        /* this is percentage change in H2 density - "change in H2 abundance" */
                        double dH2_abund = 2.*fabs( hmi.H2_total - Old_H2_abund ) / hmi.H2_total;
                        /* in testing th85ism the dH2_abund did come out zero */
                        /* >>chng 03 jun 16, change d(H2) from 0.05 to 0.1, fine resolution of H2/H exposed
                         * small numerical oscillations in Solomon process */
                        /* >>chng 03 nov 22, smallest possible ratio of dr(next)/dr changed from
                         * 0.2 to 0.05, models that started almost in H2 front need to be able to sense it */
                        /* >>chng 04 mar 15, with such small possible changes in dr, only 0.05, a thermal front
                         * can easily cause large changes in H2 and T that are not due to zoning, but to the
                         * discontinuity.  make smallest change larger to prevent hald due to dr */
                        /* >>chng 05 aug 23, thermal front allowed dr to become much too small
                         * chng from 0.02 to .6 */
                        dH2_abund = SDIV(dH2_abund);
                        double drH2_abund = radius.drad*MAX2(0.2,fac/dH2_abund );
                        ostringstream oss;
                        oss << "change in H2 abundance, d(H2)/H " << scientific << setprecision(2) << dH2_abund;
                        drChoice.insert( drH2_abund, oss.str() );
                }
        }

        int mole_dr_change = -1;
        if( nzone >= 4 )
        {       
                /* check how molecular fractions of all heavy elements are chaning relative 
                 * to total gas phase abundance */
                double max_change = 0.0;
                /* >>chng 04 jun 02, upper limit had been all species but now limit to real
                 * molecules since do not want this logic to work with the ions */
                for( int i=0; i < mole_global.num_calc; ++i )
                {
                        realnum abund, 
                                abund1,
                                abund_limit;
                        if( !mole_global.list[i]->isMonatomic() && mole_global.list[i]->isIsotopicTotalSpecies() )
                        {
                                /* >>chng 03 sep 21, add CO logic */
                                /* >>chng 04 mar 30, generalize to any molecule at all */
                                /* >>chng 04 mar 31 lower limit to abund had been 0.01, change
                                 * to 0.001 to pick up approach to molecular fronts */
                                abund = 0.;
                                for( molecule::nNucsMap::iterator atom=mole_global.list[i]->nNuclide.begin();
                                          atom != mole_global.list[i]->nNuclide.end(); ++atom)
                                {
                                        long int nelem = atom->first->el->Z-1;
                                        if (dense.lgElmtOn[nelem])
                                        {
                                                abund1 = (realnum)mole.species[i].den*atom->second/SDIV(dense.gas_phase[nelem]);
                                                if (abund1 > abund)
                                                {
                                                        abund = abund1;
                                                }
                                        }
                                }
                                /* is this an ice?  need special logic for ice since density increases
                                 * exponentially when grain temp falls below sublimation temperature 
                                 * >>chng 05 dec 06 - detect changes for smaller abundances for ices
                                 * due to large changes in ice abundances */
                                if( mole_global.list[i]->lgGas_Phase )
                                {
                                        abund_limit = 1e-3f;
                                }
                                else
                                {
                                        /* this is an ice - track its abundance at lower values so that
                                         * we resolve the sublimation transition region */
                                        abund_limit = 1e-5f;
                                }

                                if( abund > abund_limit )
                                {
                                        /* the relative change in the abundance */
                                        double relative_change = 
                                                2.0 * fabs( mole.species[i].den - struc.molecules[nzone-3][i] ) /
                                                SDIV ( mole.species[i].den + struc.molecules[nzone-3][i] ) ;
                                        if (relative_change > max_change)
                                        {
                                                max_change = relative_change;
                                                mole_dr_change = i;
                                        }
                                }
                        }
                }
                if( mole_dr_change >= 0 )
                {
                        double dr_mole_abund = radius.drad * MAX2(0.6, 0.05/SDIV(max_change));
                        ostringstream oss;
                        oss << "change in abundance species=" << mole_global.list[mole_dr_change]->label << ", ";
                        oss << scientific << setprecision(2);
                        oss << "old=" << struc.molecules[nzone-3][mole_dr_change] << " new=" << mole.species[mole_dr_change].den;
                        drChoice.insert( dr_mole_abund, oss.str() );
                }
        }

        /* some consideration due to big H2 molecule */
        for( diatom_iter diatom = diatoms.begin(); diatom != diatoms.end(); ++diatom )
                drChoice.insert( (*diatom)->H2_DR(), "change in big H2 Solomon rate line opt depth" );

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* can't make drmax large deep in model since causes feedback
         * oscillations with changes in heating or destruction rates
         * >>chng 96 oct 15, change from 5 to 10 */
        /* >>chng 96 nov 11, had been 4 * drad up to 11, change to following
         * to be similar to c90.01, max of 1.3 between 5 and 11 
         * >>chng 04 oct 29  geometry.DirectionalCosin was ioncorrect applied to this factor  */
        double drmax = ( nzone < 5 ) ? 4.*radius.drad : 1.3*radius.drad;
        drChoice.insert( drmax, "DRMAX" );

        /* time dependent recombination - conditions become very homogeneous and 
         * crash into I fronts - use last iteration's  radius as guess of current case */
        double recom_dr_last_iter = BigRadius;
        if( dynamics.lgTimeDependentStatic && dynamics.lgRecom )
        {
                /* first time through nzone == 1 */
                static long int nzone_recom = -1;
                if( nzone <= 1 )
                {
                        /* initialization case */
                        nzone_recom = 0;
                }
                else if( radius.depth < struc.depth_last[struc.nzonePreviousIteration-1] && 
                        nzone_recom < struc.nzonePreviousIteration )
                {
                        ASSERT( nzone_recom>=0 && nzone_recom<struc.nzonePreviousIteration );
                        /* case where we are within previous computed structure 
                         * first possibly increase nzone_recom so that it points deeper
                         * than this zone */
                        while( struc.depth_last[nzone_recom] < radius.depth &&
                                nzone_recom < struc.nzonePreviousIteration-1 )
                                ++nzone_recom;
                        recom_dr_last_iter = struc.drad_last[nzone_recom]*3.;
                        drChoice.insert( recom_dr_last_iter,
                                                                    "previous iteration recom logic" );
                }
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* change in electron density - radius_next keys from change in elec den,
         * remember old electron density during first call */
        /* this is low ionization solution */
        if( nzone > 2 )
        {
                /* next is-2 since nzone is on physics not c scale, and we want previous zone */
                double Efrac_old = struc.ednstr[nzone-3]/struc.gas_phase[nzone-3][ipHYDROGEN];
                double Efrac_new = dense.eden/dense.gas_phase[ipHYDROGEN];
                double dEfrac = fabs(Efrac_old-Efrac_new) / Efrac_new;
                double drEfrac;
                if( dEfrac > SMALLFLOAT )
                {
                        double fac = 0.04;
                        /* >>chng 03 dec 25, use smaller rel change in elec frac when most elec in ipMole or grains */
                        /* >>chng 04 sep 14, change to from from metals but comment out */
                        /* >>chng 04 sep 17, change to from from metals - uncomment */
                        if( dense.eden_from_metals > 0.5 )
                        {
                                fac = 0.04;
                        }
                        /* >>chng 04 feb 23, add test on hydrogen being predomintly
                         * recombined due to three-body recom, which is very sensitive
                         * to the electron density - but only do this in partially ionized medium */
                        else if( iso_sp[ipH_LIKE][ipHYDROGEN].RecomCollisFrac > 0.8 && 
                                dense.xIonDense[ipHYDROGEN][1]/dense.gas_phase[ipHYDROGEN] > 0.1 &&
                                dense.xIonDense[ipHYDROGEN][1]/dense.gas_phase[ipHYDROGEN] < 0.8 )

                        {
                                fac = 0.02;
                        }
                        /* >>chng 04 sep 17, change to 0.1 from 0.2 */
                        drEfrac = radius.drad*MAX2(0.1,fac/dEfrac);
                }
                else
                {
                        drEfrac = BigRadius;
                }
                ostringstream oss;
                oss << "change in elec den, rel chng: " << scientific << setprecision(3) << dEfrac;
                oss << " cur=" << Efrac_new << " old=" << Efrac_old;
                drChoice.insert( drEfrac, oss.str() );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* do not let thickness get too large */
        if( nzone > 20 )
        {
                /* >>chng 02 nov 05, change from 1/20 to 1/10 wasted zones early on */
                drChoice.insert( radius.depth/10., "relative depth" );
        }

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* case where stopping thickness or edge specified, need to approach slowly */
        double thickness_total = BigRadius;
        double DepthToGo = BigRadius;
        if( StopCalc.HColStop < 5e29 )
        {
                double coleff = SDIV( dense.gas_phase[ipHYDROGEN]*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.HColStop-colden.colden[ipCOL_HTOT]) / coleff );
                /* >>chng 03 oct 28, forgot to div col den above by eff density */
                thickness_total = MIN2(thickness_total , StopCalc.HColStop / coleff );
        }

        if( StopCalc.colpls < 5e29 )
        {
                double coleff = (double)SDIV( (dense.xIonDense[ipHYDROGEN][1])*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.colpls-findspecieslocal("H+")->column) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.colpls / coleff );
        }

        if( StopCalc.col_h2 < 5e29 )
        {
                /* >>chng 03 apr 15, add this molecular hydrogen */
                double coleff = (double)SDIV( hmi.H2_total*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.col_h2-findspecieslocal("H2")->column-findspecieslocal("H2*")->column) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.col_h2 / coleff );
        }

        if( StopCalc.col_h2_nut < 5e29 )
        {
                /* neutral column density of H, 2 H_2 + H^0 */
                double coleff = (double)SDIV( (2*hmi.H2_total+dense.xIonDense[ipHYDROGEN][0])*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.col_h2_nut-(2*(findspecieslocal("H2")->column+findspecieslocal("H2*")->column)+findspecieslocal("H")->column)) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.col_h2_nut / coleff );
        }

        if( StopCalc.col_H0_ov_Tspin < 5e29 )
        {
                /* >>chng 05 jan 09, add n(H0)/Tspin */
                double coleff = (double)SDIV( dense.xIonDense[ipHYDROGEN][0] / hyperfine.Tspin21cm*geometry.FillFac );
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.col_H0_ov_Tspin - colden.H0_ov_Tspin) / coleff  );
                thickness_total = MIN2(thickness_total , StopCalc.col_H0_ov_Tspin / coleff );
        }

        if( StopCalc.col_monoxco < 5e29 )
        {
                /* >>chng 03 apr 15, add this, CO */
                double coleff = (double)SDIV( (findspecieslocal("CO")->den)*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.col_monoxco-findspecieslocal("CO")->column) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.col_monoxco / coleff );
        }

        if( StopCalc.col_species < 5e29 )
        {
                /* arbitrary species */
                static molezone *SpeciesCurrent;
                static bool lgMustFind=true;
                if( lgMustFind )
                {
                        lgMustFind = false;
                        SpeciesCurrent= findspecieslocal_validate(StopCalc.chSpeciesColumn.c_str());
                }
                double coleff = (double)SDIV( (SpeciesCurrent->den)*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.col_species-SpeciesCurrent->column) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.col_species / coleff );
        }

        if( StopCalc.colnut < 5e29 )
        {
                double coleff = (double)SDIV( (dense.xIonDense[ipHYDROGEN][0])*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.colnut - findspecieslocal("H")->column) / coleff );
                thickness_total = MIN2(thickness_total , StopCalc.colnut / coleff );
        }

        /* this is case where outer radius is set */
        if( iterations.StopThickness[iteration-1] < 5e29 )
        {
                thickness_total = MIN2(thickness_total , iterations.StopThickness[iteration-1] );
                DepthToGo = MIN2(DepthToGo ,
                        iterations.StopThickness[iteration-1] - radius.depth );
        }

        /* this is case where stopping optical depth was specified */
        if( StopCalc.iptnu != rfield.nflux_with_check )
        {
                /* end optical depth has been specified */
                double dt = SDIV(opac.opacity_abs[StopCalc.iptnu-1]*geometry.FillFac);
                DepthToGo = MIN2(DepthToGo ,
                        (StopCalc.tauend-opac.TauAbsGeo[0][StopCalc.iptnu-1])/dt);
        }

        if( DepthToGo <= 0. )
                TotalInsanity();

        /* set dr if one of above tests have triggered */
        if( DepthToGo < BigRadius )
        {
                double depth_min = MIN2( DepthToGo , thickness_total/100. );
                DepthToGo = MAX2( DepthToGo / 10. , depth_min );
                /* want to
                
                approach the outer edge slowly,
                 * the need for this logic is most evident in brems.in - 
                 * HI fraction varies across coronal model */
                double drThickness = MIN2( thickness_total/10. , DepthToGo );
                drChoice.insert( drThickness, "depth to go" );
        }

        /* stop AV - usually this is dust, but we consider all opacity sources,
         * so always include this */
        /* compute some average grain properties */
        double AV_to_go = BigRadius;
        if( rfield.opac_mag_V_extended > SMALLFLOAT && rfield.opac_mag_V_point > SMALLFLOAT )
        {
                double SAFETY = 1. + 10.*DBL_EPSILON;
                /* by default stop av is very large, and opacity can be very small, so ratio
                 * goes to inf - work with logs to see how big the number is */
                /* apply safety margin to avoid updates to the total extinction getting lost
                 * in the numerical precision */
                double ave = log10(SAFETY*StopCalc.AV_extended - rfield.extin_mag_V_extended) - 
                        log10(rfield.opac_mag_V_extended);
                double avp = log10(SAFETY*StopCalc.AV_point - rfield.extin_mag_V_point) - 
                        log10(rfield.opac_mag_V_point);
                AV_to_go = MIN2( ave , avp );
                if( AV_to_go < 37. )
                {
                        AV_to_go = exp10( AV_to_go)/geometry.FillFac;
                        /* this is to make sure that we go slightly deeper than AV so that
                         * we trigger this stop */
                        AV_to_go *= 1.0001;
                }
                else
                        AV_to_go = BigRadius;
                /*fprintf(ioQQQ,"DEBUG next dr %.3e %.3e %.3e\n", AV_to_go , ave, avp );*/
        }
        drChoice.insert( AV_to_go, "A_V to go" );

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        /* spherical models, do not want delta R/R big */
        double drSphere = radius.Radius*0.04;
        drChoice.insert( drSphere, "sphericity" );

        /* optical depth to electron scattering */
        double dRTaue = radius.drChange/(dense.eden*6.65e-25*geometry.FillFac);
        /* allow larger dr when constant temperature,
         * to prevent some ct models from taking too many cells */
        if( thermal.lgTemperatureConstant ) 
                dRTaue *= 3.;
        drChoice.insert( dRTaue, "optical depth to electron scattering" );

        /*>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>*/
        if( dense.flong != 0. )
        {
                double drFluc = PI2/10./2./dense.flong;
                /* >>chng 04 sep 18, caused cautions that ionization jumps occurred.
                 * set to have the value */
                drFluc /= 2.;
                drChoice.insert( drFluc, "density fluctuations" );
        }

        /* keep dr constant in first two zones, if it wants to increase,
         * but always allow it to decrease.
         * to guard against large increases in efrac for balmer cont photo dominated models,
         */
        double drStart = ( nzone <= 1 ) ? radius.drad : BigRadius;
        drChoice.insert( drStart, "capped to old DR in first zone" );

        // lgSdrmaxRel true if sdrmax is relative to current radius, false if limit in cm
        double rfacmax = radius.lgSdrmaxRel ? radius.Radius : 1.;
        drChoice.insert( rfacmax*radius.sdrmax, "sdrmax" );

        /* >>chng 05 aug 05, in case of thermal front, where temp is falling quickly and
         * conditions change very fast, the zone thickness does not really affect the change
         * in conditions and can cause zoning to become very very thin, which causes
         * an abort.  this occurs between 200 and 1000K.  if we are doing temp soln,
         * temp is between these values, and temp is changing rapidly, do not make sone
         * thickness much smaller */
        /* do not use thermal front logic in case of recombination time dep cloud */
        if( nzone >= 5 && !dynamics.lgTimeDependentStatic )
        {
                /* >>chng 05 aug 23, upper bound of thermal from from 1000K to 4000K */
                /*if( phycon.te > 200. && phycon.te < 1000. && */
                if( phycon.te > 200. && phycon.te < 3000. && 
                        /* >>chng 05 aug 23, from > 10% in zone to to two zones > 5%,
                         * to fix leiden v3 with large H2 */
                        (struc.testr[nzone-3] - phycon.te)/phycon.te > 0.02 &&
                        (struc.testr[nzone-4] - phycon.te)/phycon.te > 0.02 &&
                        (struc.testr[nzone-5] - phycon.te)/phycon.te > 0.02 )
                {
                        /* the 0.91 is to make dr unique, so that print statement that
                         * follows will identify this reason */
                        double drThermalFront = radius.drad * 0.91;
                        drChoice.clear();
                        drChoice.insert( drThermalFront, "thermal front logic" );
                }
        }

        ASSERT( drChoice.size() > 0 );

        /* dr = zero is a logical mistake */
        if( drChoice.begin()->first <= 0. )
        {
                drList::const_iterator ptr = drChoice.begin();
                fprintf( ioQQQ, " radius_next finds insane drNext: %.2e\n", ptr->first );
                fprintf( ioQQQ, " all drs follow:\n" );
                while( ptr != drChoice.end() )
                {
                        fprintf( ioQQQ, " %.2e %s\n", ptr->first, ptr->second.c_str() );
                        ++ptr;
                }
                cdEXIT(EXIT_FAILURE);
        }

        /* option to force min drad relative to depth */
        /* see google group thread "[cloudy-dev] sdrmin_rel_depth oscillation" ~2012/6/13 for discussion
         * of validity of this test */
        if( !radius.lgFixed && drChoice.begin()->first < radius.depth*radius.sdrmin_rel_depth )
        {
                drChoice.clear();
                drChoice.insert( radius.depth*radius.sdrmin_rel_depth, "sdrmin_rel_depth" );
        }

        /* option to force min drad */
        double rfacmin = radius.lgSdrminRel ? radius.Radius : 1.;
        if( drChoice.begin()->first < rfacmin*radius.sdrmin )
        {
                drChoice.clear();
                drChoice.insert( rfacmin*radius.sdrmin, "sdrmin" );
        }

        /* this is general code that prevents zone thickness drNext from
         * becoming too thin, something that can happen for various bad reasons
         * HOWEVER we do not want to do this test for certain density laws,
         * for which very small zone thicknesses are unavoidable
         * the special cases are:
         * special density law,
         * globule density law,
         * carbon +-0 i front
         * the fluctuations command
         * drMinimum was set in radius_first to either sdrmin (set drmin) or
         * some fraction of the initial radius - it is always set
         * to something non-trivial.  
         * sdrmin is set with the "set dr" command - is SMALLFLOAT by default */
        if( strcmp(dense.chDenseLaw,"DLW1") != 0 && 
            strcmp(dense.chDenseLaw ,"GLOB") != 0 && 
            dense.flong == 0. &&
            // stopping on depth to go is not a fault
            drChoice.begin()->first != DepthToGo &&
                // stopping on Av to go is not a fault
                drChoice.begin()->first != AV_to_go )
        {
                fixit( "drMinimum does not get reevaluated in each iteration,"
                        " so time-dependent solutions just use the first drad and Hden."
                        " Re-eval here for now to keep this minimum from blowing." );
                radius.drMinimum = (realnum)(radius.drad * dense.gas_phase[ipHYDROGEN]/1e7);

                /* don't let dr get smaller than drMinimum, if this resets drNext
                 * then code stops with warning that zones got too thin
                 * this can happen due to numerical oscillations, which the code
                 * tries to damp out by making the zones thinner.
                 * scale by radius.Radius/radius.rinner to stop very spherical 
                 * simulations from false trigger on dr too small */
                /* drMinimum is drad * hden, to make it proportional to optical depth.
                 * This avoids false trigger across thermal fronts. */
                if( drChoice.begin()->first*radius.Radius/radius.rinner*dense.gas_phase[ipHYDROGEN] < radius.drMinimum )
                {
                        radius.drNext = radius.drMinimum/dense.gas_phase[ipHYDROGEN];
                        /* leaving this at true will cause the model to stop with a warning
                         * that the zone thickness is too small */
                        radius.lgDrMinUsed = true;
                        /* set abort handler */
                        lgAbort = true;
                        /* must decrement nzone, since we will not complete this zone, and will not have
                         * valid structure data for it */
                        --nzone;
                        fprintf( ioQQQ, 
                                "\n DISASTER PROBLEM radius_next finds dr too small and aborts.  "
                                "This is zone %ld iteration %ld.  The thickness was %.2e\n", 
                                nzone, 
                                iteration,
                                radius.drNext);
                        fprintf( ioQQQ, 
                                " If this simulation seems reasonable you can override this limit "
                                "with the command SET DRMIN %.2e\n\n", 
                                radius.drNext*2);
                        /* set abort flag */
                        lgAbort = true;
                        return 1;
                }
        }

        /* factor to allow for slop in floating numbers */
        const double Z = 1.0001;

        /* following is to make thickness of model exact
         * n.b., on last zone, drNext can be NEGATIVE!!
         * DEPTH was incremented at start of zone calc in newrad,
         * has been outer edge of zone all throughout */
        double drOuterRadius = (iterations.StopThickness[iteration-1]-radius.depth)*Z;
        drChoice.insert( drOuterRadius, "outer radius" );

        if( cosmology.lgDo )
        {
                drChoice.clear();
                double drHz = SPEEDLIGHT / GetHubbleFactor( cosmology.redshift_current );
                drChoice.insert( drHz, "c over H(z)" );
        }

        // choose the smallest dR as the next choice
        radius.drNext = drChoice.begin()->first;

        /* set choice flag if provided -- in practice, just for rt.lgMaserSetDR */
        drChoice.begin()->second.select( );

        /* all this is to only save on last iteration
         * the save dr command is not really a save command, making this necessary
         * lgDRon is set true if "save dr" entered */
        /* lgDRPLst was set true if "save" had "last" on it */
        bool lgDoPun = ( save.lgDROn && !( save.lgDRPLst && !iterations.lgLastIt ) );
        if( (trace.lgTrace && trace.lgDrBug) || lgDoPun )
        {
                fprintf( save.ipDRout , "%ld\t%.5e\t%.3e\t%.3e\t%s\n", nzone, radius.depth,
                         radius.drNext, radius.Depth2Go, drChoice.begin()->second.c_str() );
        }

        ASSERT( radius.drNext > 0. );

        if( trace.lgTrace )
        {
                fprintf( ioQQQ, " radius_next chooses next drad drNext=%.4e; this drad was %.4e\n", 
                         radius.drNext, radius.drad );
        }

        return 0;
}

/*ContRate called by radius_next to find energy of maximum continuum-gas interaction */
STATIC void ContRate(double *freqm, 
                     double *opacm)
{
        long int i, 
          ipHi,
          iplow, 
          limit;
        double FreqH, 
          Freq_nonIonizing, 
          Opac_Hion, 
          Opac_nonIonizing, 
          Rate_max_Hion, 
          Rate_max_nonIonizing;

        DEBUG_ENTRY( "ContRate()" );

        /* 
         * find maximum continuum interaction rate,
         * these should be reset in following logic without exception,
         * if they are still zero at the end we have a logical error 
         */
        Rate_max_nonIonizing = 0.;
        Freq_nonIonizing = 0.;
        Opac_nonIonizing = 0.;

        /* this must be reset to val >= 0 */
        *opacm = -1.;
        *freqm = -1.;

        /* do up to carbon photo edge if carbon is turned on */
        /* >>>chng 00 apr 07, add test for whether element is turned on */
        if( dense.lgElmtOn[ipCARBON] )
        {
                /* carbon is turned on, use carbon 1 edge */
                ipHi = Heavy.ipHeavy[ipCARBON][0] - 1;
        }
        else
        {
                /* carbon turned off, use hydrogen Balmer continuum */
                ipHi = iso_sp[ipH_LIKE][ipHYDROGEN].fb[ipH2s].ipIsoLevNIonCon-1;
        }

        /* start at plasma frequency */
        for( i=rfield.ipPlasma; i < ipHi; i++ )
        {
                /* this does not have grain opacity since grains totally passive
                 * at energies smaller than CI edge */
                if( rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*(opac.opacity_abs[i] - 
                  gv.dstab[i]*dense.gas_phase[ipHYDROGEN]) > Rate_max_nonIonizing )
                {
                        Rate_max_nonIonizing = rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*
                          (opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN]);
                        Freq_nonIonizing = rfield.anu(i);
                        Opac_nonIonizing = opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN];
                }
        }

        /* not every continuum extends beyond C edge-this whole loop can add to zero
         * use total opacity here
         * test is to put in fir continuum if free free heating is important */
        if( safe_div(CoolHeavy.brems_heat_total,thermal.htot,0.0) > 0.05 )
        {
                /* this is index for energy where cloud free free optical depth is unity,
                 * is zero if no freq are opt thin */
                iplow = MAX2(1 , rfield.ipEnergyBremsThin );
        }
        else
        {
                /* >>>chng 00 apr 07, from Heavy.ipHeavy[0][5] to ipHi defined above, since
                 * would crash if element not defined */
                iplow = ipHi;
        }
        /* do not go below plasma frequency */
        iplow = MAX2( rfield.ipPlasma , iplow );

        /* this energy range from carbon edge to hydrogen edge */
        limit = MIN2(rfield.nflux,iso_sp[ipH_LIKE][ipHYDROGEN].fb[ipH1s].ipIsoLevNIonCon-1);
        for( i=iplow; i < limit; i++ )
        {
                if( rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*(opac.opacity_abs[i] - 
                  gv.dstab[i]*dense.gas_phase[ipHYDROGEN]) > Rate_max_nonIonizing )
                {
                        Rate_max_nonIonizing = rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*
                          (opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN]);
                        Freq_nonIonizing = rfield.anu(i);
                        Opac_nonIonizing = opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN];
                }
        }

        /* variables to check continuum interactions over Lyman continuum */
        Rate_max_Hion = 0.;
        Opac_Hion = 0.;
        FreqH = 0.;

        /* not every continuum extends beyond 1 Ryd-this whole loop can add to zero */
        for( i=iso_sp[ipH_LIKE][ipHYDROGEN].fb[ipH1s].ipIsoLevNIonCon-1; i < rfield.nflux; i++ )
        {
                if( rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*(opac.opacity_abs[i] - 
                  gv.dstab[i]*dense.gas_phase[ipHYDROGEN]) > Rate_max_Hion )
                {
                        /* Rate_max_Hion = anu(i)*flux(i)/widflx(i)*opac(i) */
                        Rate_max_Hion = rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*
                          (opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN]);
                        FreqH = rfield.anu(i);
                        Opac_Hion = opac.opacity_abs[i] - gv.dstab[i]*dense.gas_phase[ipHYDROGEN];
                }
        }

        /* use Lyman continuum if its opacity is larger than non-h ion */
        if( Rate_max_nonIonizing < 1e-30  && Opac_Hion > SMALLFLOAT )
        {
                /* this happens for laser source - use Lyman continuum */
                *opacm = Opac_Hion;
                *freqm = FreqH;
        }
        /* >>chng 05 aug 03, add last test on Opac_Hion for case where we go very
         * deep and very little radiation is left */
        else if( Opac_Hion > Opac_nonIonizing && Rate_max_Hion/SDIV(Rate_max_nonIonizing) > 1e-10 && Opac_Hion > SMALLFLOAT )
        {
                /* use Lyman continuum */
                *opacm = Opac_Hion;
                *freqm = FreqH;
        }
        else
        {
                /* not much rate in the Lyman continuum, stick with low energy */
                *opacm = Opac_nonIonizing;
                *freqm = Freq_nonIonizing;
        }

#       if 0
        /*>>chng 06 aug 12, do not let zones become optically thick to
        * peak of dust emission - one of NA's vastly optically thick dust clouds
        * did not conserve total energy - very opticallly thick to ir dust emission
        * so ir was absorbed and reemitted many times
        * this makes sure the cells remain optically thin to dust emission */
        if( gv.lgDustOn() && gv.lgGrainPhysicsOn )
        {
                double fluxGrainPeak = -1.;
                long int ipGrainPeak = -1;
                long int ipGrainPeak2 = -1;

                i = 0;
                while( rfield.anu(i) < 0.0912  && i < (rfield.nflux-2) )
                {
                        /* >>chng 06 aug 23.  Only want to do this test for the IR dust continuum, therefore only 
                         * check on optical depth for wavelengths greater than 1 micron */
                        if( gv.GrainEmission[i]*rfield.anu(i)*opac.opacity_abs[i] > fluxGrainPeak )
                        {
                                ipGrainPeak = i;
                                fluxGrainPeak = gv.GrainEmission[i]*rfield.anu(i)*opac.opacity_abs[i];
                        }
                        ++i;
                }
                ASSERT( fluxGrainPeak>=0. && ipGrainPeak >=0 );

                /* >>chng 06 aug 23.  We have just found the wavelength and flux at the peak of the IR emission.
                 * Now we want to find the wavelength, short of the peak, which corresponds to 5% of the 
                 * peak emission.  This wavelength will be where the code checks to make sure the zone has
                 * not become optically thick.  Since dust opacity generally decreases with wavelength,  this assures that
                 * the optical depths are small for all wavelengths where the flux is appreciable.  Tests show that
                 * this allows for flux/luminosity conservation to within ~5% for an AV of 1e4 mag and at least 2 iterations*/
                i = ipGrainPeak;
                /* >>chng 06 aug 23.  Only want to do this test for the IR dust continuum, therefore only 
                 * check on opacities for wavelengths shortward of the peak and greater than 1 micron 
                 * this routine can be called with the dust emission totally zero - it is only evaluated 
                 * late to save time.  don't do the following tests if peak dust emission is zero */
                if( fluxGrainPeak > 0. )
                {
                        while( rfield.anu(i) < 0.0912 && i < (rfield.nflux-2) )
                        {
                                /* find wavelength where flux = 5% of peak flux, shortward of the peak */
                                if( gv.GrainEmission[i]*rfield.anu(i)*opac.opacity_abs[i] > 0.05*fluxGrainPeak )
                                {
                                        /* This will be the array number and flux at 10% of the peak */
                                        ipGrainPeak2 = i;
                                }
                                ++i;
                        }
                        ASSERT( ipGrainPeak2>=0 );

                        /* use this as a limit to the dr if opacity is larger */
                        if( opac.opacity_abs[ipGrainPeak2] > *opacm )
                        {
                                /* scale opacity by factor of 10, which further decreases the zone thickness*/
                                *opacm = opac.opacity_abs[ipGrainPeak2]*10.;
                                *freqm = rfield.anu(ipGrainPeak2);
                                /*fprintf(ioQQQ,"DEBUT opac peak %.2e %.2e \n",
                                        *opacm , *freqm );*/
                        }
                }
        }
#       endif

        {
                /* following should be set true to print contributors */
                enum {DEBUG_LOC=false};
                if( DEBUG_LOC )
                {
                        fprintf(ioQQQ,"conratedebug \t%.2e\t%.2e\t%.2e\t%.2e\t%.2e\t%.2e\t%.2e\t%.2e\n", 
                        Rate_max_nonIonizing,Freq_nonIonizing,Opac_nonIonizing,
                        Rate_max_Hion,FreqH ,Opac_Hion,*freqm,*opacm
                        );

                }
        }

        /* these were set to -1 at start, and must have been reset in one of the
         * two loops.  Logic error if still <0. */
        /* >>chng 05 aug 03, change logic to -1 on entry and check at least zero
         * here - will be zero if NO radiation field exists, perhaps since totally
         * attenuated */
        ASSERT( *opacm >= 0. && *freqm >= 0. );
        return;
}

/*GrainRateDr called by radius_next to find grain heating rate dr */
STATIC void GrainRateDr(double *grfreqm, 
                        double *gropacm)
{
        long int i, 
          iplow;
        double xMax;

        DEBUG_ENTRY( "GrainRateDr()" );

        /* in all following changed from anu2 to anu  july 25 95
         *
         * find maximum continuum interaction rate */

        /* not every continuum extends beyond C edge-this whole loop can add to zero
         * use total opacity here
         * test is to put in fir continuum if free free heating is important */
        if( safe_div(CoolHeavy.brems_heat_total,thermal.htot,0.0) > 0.05 )
        {
                /* this is pointer to energy where cloud free free optical depth is unity,
                 * is zero if no freq are opt thin */
                iplow = MAX2(1 , rfield.ipEnergyBremsThin );
        }
        else
        {
                /* do up to carbon photo edge if carbon is turned on */
                /* >>>chng 00 apr 07, add test for whether element is turned on */
                if( dense.lgElmtOn[ipCARBON] )
                {
                        /* carbon is turned on, use carbon 1 edge */
                        iplow = Heavy.ipHeavy[ipCARBON][0];
                }
                else
                {
                        /* carbon truned off, use hydrogen balmer continuum */
                        iplow = iso_sp[ipH_LIKE][ipHYDROGEN].fb[ipH2s].ipIsoLevNIonCon;
                }
        }

        xMax = -1.;
        /* integrate up to H edge */
        for( i=iplow-1; i < Heavy.ipHeavy[ipHYDROGEN][0]; i++ )
        {
                if( rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*opac.opacity_abs[i] > xMax )
                {
                        xMax = rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*
                          opac.opacity_abs[i];
                        *grfreqm = rfield.anu(i);
                        *gropacm = opac.opacity_abs[i];
                }
        }
        /* integrate up to heii edge if he is turned on,
         * this logic will not make sense if grains on but he off, which in itself makes no sense*/
        if( dense.lgElmtOn[ipHELIUM] )
        {
                for( i=Heavy.ipHeavy[ipHYDROGEN][0]-1; i < Heavy.ipHeavy[ipHELIUM][1]; i++ )
                {
                        if( rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*opac.opacity_abs[i] > xMax )
                        {
                                xMax = rfield.anu(i)*rfield.flux[0][i]/rfield.widflx(i)*
                                  opac.opacity_abs[i];
                                *grfreqm = rfield.anu(i);
                                *gropacm = opac.opacity_abs[i];
                        }
                }
        }

        /* possible that there is NO ionizing radiation, in extreme cases,
         * if so then xMax will still be negative */
        if( xMax <= 0. )
        {
                *gropacm = 0.;
                *grfreqm = 0.;
        }
        return;
}