Nonequilibrium free-energy calculations of fluids using LAMMPS

Abstract

We present a guide to compute the absolute free energies of classical fluids using nonequilibrium free-energy techniques within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The main approach is based on the construction of a thermodynamic path connecting the fluid of interest to either atomic or molecular variants of the Uhlenbeck-Ford (UF) model as reference systems. We describe these reference systems in detail, discuss their implementation in the LAMMPS package and make available source code, scripts as well as auxiliary files. As an illustration we detail a number of distinct applications, involving systems characterized by fundamentally different interactions. In addition to two different atomic models (mW water and the MEAM-2NN CuZr liquid binary alloy), we consider three molecular models for water, two of them rigid (TIP4P and SPC/E) and one flexible (q-SPC/Fw). For the molecular models we develop UF-based reference systems for which the free energies are given by a sum of two contributions: an intermolecular part described by the known UF free energy and an intramolecular contribution that can be determined analytically. The tools described in this paper provide a platform on which fluid-phase free energies can be easily and efficiently computed using the LAMMPS code. In addition to being useful for the development of new models for liquid phases, the tools may also find applications in the construction of community databases containing thermodynamic properties of existing models.