[lammps-users] Enthalpy of Vaporization calculation

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Hello LAMMPS users,

I would like to calculate the enthalpy of vaporization of a molecule using the cohesive energy density (E_CED) with the following equation:

Dhv = E_CEDMW/rho + RT

Where Dhv is the enthalpy of vaporization in kcal/mol, E_CED is the cohesive energy density in kcal/m^3, MW is the molecular weight of the molecule in kg/mol, rho is the density of the simulation in kg/m^3, R is the universal gas constant, and T is the temperature.

E_CED is defined as the total intermolecular nonbond energy per unit volume. I interpret this as the intermolecular evdwl + ecoul + elong. Is this correct? If so, is there a way to break down the evdwl to its intramolecular and intermolecular components?

Thanks,

Tim Kokan

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Hi Tim. Your interpretation of E_CED sounds correct to me. And I agree that nonbond energy is “evdwl + ecoul + elong”. You do have that problem of including the intramolecular nonbond interactions. We have a ninteraction energy fix that should be released soon that would enable you to compute the interaction between individual molecules and their environment. This should give you what you want. For now, you’d have to compute the interactions as a post-process step. -Paul