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We evaluate the accuracy of varying thermal expansion models for the quasi-harmonic approximation (QHA) relative to molecular dynamics (MD) for 10 sets of enantiotropic organic polymorphs. Relative to experiment we find that MD, using an off-the-shelf point charge potential gets the sign of the enthalpic contributions correct for 6 of the 10 pairs of polymorphs and the sign of the entropic contributions correct for all pairs. We find that anisotropic QHA provides little improvement to the error in free energy differences from MD relative to isotropic QHA, but does a better job capturing the thermal expansion of the crystals. A form of entropy-enthalpy compensation allows the free energy differences of QHA to deviate less than 0.1 kcal/mol from MD for most polymorphic pairs, despite errors up to 0.4 kcal/mol in the entropy and enthalpy. Deviations in the free energy of QHA and MD do not clearly correlate with molecular flexibility, clarifying a previously published finding. Much of the error previously fou...

Adding Anisotropy to the Standard Quasi-Harmonic Approximation Still Fails in Several Ways to Capture Organic Crystal Thermodynamics