Abstract

Quantitative estimations of thermodynamic stabilities – measured by free energies – must take into account thermal and zero-point energy fluctuations. While these effects are easily estimated within a harmonic approximation, corrections arising from the anharmonic nature of the interatomic potential are often crucial and require expensive path integral simulations. Consequently, different approximate methods for computing affordable estimates of anharmonic free energies have been developed. Understanding which of the approximations involved are justified for a given system is complicated by the lack of comparative benchmarks. To facilitate this choice we assess the ac- curacy of some of the commonly used approximate methods: vibrational self-consistent field and self-consistent phonons. We compare anharmonic corrections to Helmholtz free energies against ref- erence path integral calculations for a diverse set of systems, ranging from simple weakly anharmonic solids to flexible molecular crystals with freely-rotating units [1]. We conclude that efforts towards obtaining computationally-feasible anharmonic free-energies of molecular systems should be directed towards reducing the expense of path integral methods, while the approximate methods should be used as an effective sampling approach to generate data to train and validate machine learning potentials that give access to highly accurate and low cost ab initio interatomic interactions.

[1] Venkat Kapil, Edgar Engel, Mariana Rossi, and Michele Ceriotti. Assessment of Approxi- mate Methods for Anharmonic Free Energies. Journal of Chemical Theory and Computation, 15(11):5845–5857, November 2019.