Hi LAMMPS users,

I am posting this question again to see if anyone has any new help with my problem:

Has anyone successfully used LAMMPS to calculate the sensible enthalpy of a substance? I am attempting to calculate sensible enthalpy for a propellant I am researching, but I am getting calculated results that are up to 50% off. My simulations predict the condensed-phase density of the substance accurately, but are significantly off on enthalpy.

I have also tried a simple argon example, but am still significantly off on my enthalpy change prediction.

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: = <U*N> + <P/rho>. I have also tried = <U*N + P/rho>.

Where H is the computed enthalpy, U is the total energy/atom (using ‘thermo_modify norm yes’), N is the number of atoms per molecule – for argon N=1, P is the pressure calculated by LAMMPS and rho is the density calculated by LAMMPS.

I have included the long-range pressure correction for the LJ term and the long-range pressure and energy corrections for the LJ and coulombic terms. I have tested out these long-range corrections to make sure they work correctly and they seem to be working fine.

I perform two different P-T combinations and calculate the difference in enthalpy between the two runs. This difference is typically 30-50% larger than experimental values even when the predicted densities for the two runs are very close to the experimental values. When the substances are in the gas phase, the enthalpy difference is very accurate (when the KE dominates the total energy term and there is very little PE). However, when the PE term becomes more significant (in the condensed phases), my enthalpy change predictions are incorrect.

I have tried using both ‘real’ and ‘lj’ units with no success.

Any help would be appreciated.

Thanks,

Tim Kokan