Implications of the volume dependent convergence of anharmonic free energy methods

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Abstract
The free energies calculated by lattice dynamics in the quasiharmonic approximation and anharmonic perturbation theory are compared with exact results obtained by Monte Carlo simulation. The free energies obtained with the approximate methods are more accurate at smaller volumes. One implication of this result is that inclusion of interactions beyond nearest neighbor increases the accuracy of these methods due to the compression associated with the attractive longer range interactions. Also, these methods will be accurate to higher temperatures when thermal expansion is smaller, as is shown to be the case for the Morse crystal as compared to the Lennard-Jones crystal.