[lammps-users] Energy Units Question

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

Tim, I don’t have a good answer for you on this one. Has anyone successfully predicted benzene’s internal energy changes using that force field? I’m not so confident that compass will yield the right answers for changes in benzene’s internal energy. Especially the delta values may be sensitive to FF inadequacies. As far as how you’re doing the calculation, your approach makes sense to me. -Paul