(476f) A Free-Energy Diabat Approach to Polymorph Stability

Many molecules can crystallize into several different crystal structures (polymorphs), each with different solubility and growth/dissolution kinetics. There are several methods to discover and predict the free-energies of polymorphs. All the methods, however, rely on quasi-harmonic approximations or on expensive multi-step simulations. We propose a framework that combines the idea of Bennett’s diabat interpolation method and energy gaps from lattice-switch Monte Carlo techniques previously used only in electron transfer literature. We illustrate that only two unbiased simulations are required for the Helmholtz free-energy difference between FCC and BCC polymorphs of Gaussian core solid. The method is generalized for the Gibbs free-energy difference in the isothermal-isobaric ensemble. The relevant Zwanzig-Bennett relation in the NPT ensemble is applicable for the diabats of triclinic polymorphs. The simple interpolation idea is not very accurate for anharmonic diabats, however the two unbiased simulations, do provide preliminary bias potentials for umbrella sampling. The flattened landscape can then be efficiently sampled. We illustrate the method for zirconium (BCC and HCP phases) and benzene molecular crystals (form I and form III).