Free energy estimates are key to quantitatively understanding physical-chemical processes ranging from solubility to binding affinity. In the case of polymorphism of molecular crystals in particular, accurate and efficient lattice free energies are needed to determine the relative stability of crystal packings, which is important in many industries, including chemicals, pharmaceuticals, semiconductors, and food products. An accurate prediction of the relative thermodynamic stability of polymorphs at different temperatures requires calculating the entropic contribution to free energy.