Phase-Transferable Force Field for Alkali Halides.

A longstanding goal of computational chemistry is to predict the state of materials in all phases with a single model. This is particularly relevant for materials that are difficult or dangerous to handle or compounds that have not yet been created. Progress toward this goal has been limited, as most work has concentrated on just one phase, often determined by particular applications. In the framework of the development of the Alexandria force field, we present here new polarizable force fields for alkali halides with Gaussian charge distributions for molecular dynamics simulations. We explore different descriptions of the van der Waals interaction, like the commonly applied 12-6 Lennard-Jones (LJ), and compare it to "softer" ones, such as the 8-6 LJ, Buckingham, and a modified Buckingham potential. Our results for physicochemical properties of the gas, liquid, and solid phases of alkali halides are compared to experimental data and calculations with reference polarizable and nonpolarizable force fields. The new polarizable force field that employs a modified Buckingham potential predicts the tested properties for gas, liquid, and solid phases with a very good accuracy. In contrast to reference force fields, this model reproduces the correct crystal structures for all alkali halides at low and high temperature. Seeing that experiments with molten salts may be tedious due to high temperatures and their corrosive nature, the models presented here can contribute significantly to our understanding of alkali halides in general and melts in particular.