Has anyone successfully used LAMMPS to calculate sensible enthalpy and entropy of a substance? I am attempting to calculate sensible enthalpy for a propellant I am researching, but I am getting calculated results that are an order of magnitude off. My simulations predict the condensed-phase density of the substance fairly accurately, but are significantly off on enthalpy.

I am calculating the enthalpy of a substance (using an NPT run) by getting the average E_total (from the LAMMPS output) and adding the average pressure divided by the average density (h = u + P/rho). I do this for two different P-T combinations and calculate the difference in enthalpy between the two runs. This difference is typically almost an order of magnitude (or more) too small as compared with experimental data.

I assume that the units on E_total are in kcal/mol, the units on temperature are in K, the units on pressure are in atmospheres, and the units on volume are in cubic angstroms. In my input file I set the units to “real”.

Any help would be appreciated.

Thanks,

Tim Kokan