Index: mole_h2.cpp
===================================================================
--- mole_h2.cpp	(revision 12850)
+++ mole_h2.cpp	(working copy)
@@ -674,8 +674,10 @@
 			if(lgDeBug)fprintf(ioQQQ,"\t%.1e",CollRate_levn[ihi][ilo]);
 
 			/* now get upward excitation rate - units s-1 */
+			// use dsexp() explicitly rather than divide Boltzmann() factors
+			// to avoid problems with underflow at low Te (see ticket #284)
 			CollRate_levn[ilo][ihi] = CollRate_levn[ihi][ilo]*
-				safe_div(states[ihi].Boltzmann(),states[ilo].Boltzmann(),0.)*
+				dsexp( (states[ihi].energy().K() - states[ilo].energy().K()) / phycon.te )*
 				states[ihi].g() / states[ilo].g();
 		}
 		if(lgDeBug)fprintf(ioQQQ,"\n");
@@ -691,8 +693,10 @@
 			if(lgDeBug)fprintf(ioQQQ,"\t%.1e",ratein);
 
 			/* now get upward excitation rate */
+			// use dsexp() explicitly rather than divide Boltzmann() factors
+			// to avoid problems with underflow at low Te (see ticket #284)
 			double rateout = ratein *
-				safe_div(states[ihi].Boltzmann(),states[ilo].Boltzmann(),0.0)*
+				dsexp( (states[ihi].energy().K() - states[ilo].energy().K()) / phycon.te )*
 				states[ihi].g()/states[ilo].g();
 
 			/* these are general entries and exits going into vector */
@@ -2055,7 +2059,7 @@
 		long iRotHi = ipRot_H2_energy_sort[ipHi];
 
 		realnum H2stat = states[ipHi].g();
-		double H2boltz = states[ipHi].Boltzmann();
+		double EhiK = states[ipHi].energy().K();
 
 		for( long ipLo=0; ipLo<ipHi; ++ipLo )
 		{
@@ -2065,9 +2069,11 @@
 			/* collision de-excitation [s-1] */
 			realnum colldn = H2_X_coll_rate[ipHi][ipLo];
 			/* inverse, rate up, [cm-3 s-1] */
+			// use dsexp() explicitly rather than divide Boltzmann() factors
+			// to avoid problems with underflow at low Te (see ticket #284)
 			realnum collup = colldn *
 				H2stat / states[ipLo].g() *
-				safe_div(H2boltz, states[ipLo].Boltzmann(), 0.0 );
+				dsexp( (EhiK - states[ipLo].energy().K()) / phycon.te );
 			
 			H2_col_rate_out[iVibHi][iRotHi] += colldn;
 			H2_col_rate_in[iVibLo][iRotLo]  += colldn * H2_old_populations[0][iVibHi][iRotHi];