Hello LAMMPS users,

I have a fairly fundamental question about the calculation of internal energy in LAMMPS.

I have a simulation of 125 benzene molecules and would like to compute the average internal energy of a given benzene molecule. I am calculating the average internal energy with the following equation:

u = *12

where is the average total energy (pe+ke) in units of kcal/mol of atoms. The potential energy is emol+evdwl+ecoul+elong. I multiply by 12 because benzene has 12 atoms per molecule and I have set “thermo_modify norm yes”. As I understand it, the resulting internal energy should be in units of kcal/mol of benzene molecules. Is this correct?

Using this method yields a change in internal energy which is significantly different from experimental values (almost 2.5 times larger than experimental). I am using the COMPASS force-field model which uses benzene as one of its training set molecules. I would expect the COMPASS force-field model to do a fairly accurate job of predicting the internal energy.

Does anyone have any suggestions for what I may be doing wrong? Any help would be appreciated.

Tim Kokan