Mold: a LAMMPS package to compute interfacial free energies and nucleation rates

Abstract

The determination of the free energy cost of forming an interface between two phases, i.e. the interfacial free energy, is critical for characterizing a phase transition. In the particular case of liquid-to-solid transitions, it is not straightforward to obtain this quantity through experiment or computation. Here, we present the computational package Mold, which is integrated in the Molecular Dynamics open-source software LAMMPS (Thompson et al., 2022). Mold enables direct calculation of the interfacial free energy between arbitrarily complex crystal structures and liquids/solutions at coexistence conditions through the Mold Integration method (Espinosa et al., 2014). Furthermore, the extension of this method for determining crystal nucleation rates—the probablity of a critical nucleus to emerge per unit of volume and time within a metastable phase—is also incorporated in the package, termed Lattice Mold method (Espinosa, Sampedro, et al., 2016). Altogether, we detail here the required codes, scripts, and instructions for evaluating the two most challenging quantities determining the feasibility of a liquid-to-solid transition, implemented to work alongside the MD package LAMMPS.