Nonequilibrium free-energy calculation of solids using LAMMPS

TL;DR

This paper introduces nonequilibrium MD techniques implemented in LAMMPS for efficient free-energy calculations of solids, offering an alternative to traditional methods and demonstrated on iron polymorphic transitions.

Contribution

The paper presents novel nonequilibrium free-energy calculation methods integrated into LAMMPS, improving efficiency over standard equilibrium approaches.

Findings

Methods successfully compute free-energy differences between Hamiltonians.

Application to iron polymorphs demonstrates practical utility.

Guidelines for optimizing process parameters are provided.

Abstract

This article describes nonequilibrium techniques for the calculation of free energies of solids using molecular dynamics (MD) simulations. These methods provide an alternative to standard equilibrium thermodynamic integration methods and often present superior efficiency. Here we describe the implementation in the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code of two specific nonequilibrium processes that allow the calculation of the free-energy difference between two different system Hamiltonians as well as the free-energy temperature dependence of a given Hamiltonian, respectively. The theory behind the methods is summarized, and we describe (including fragments of LAMMPS scripts) how the process parameters should be selected to obtain the best-possible efficiency in the calculations of free energies using nonequilibrium MD simulations. As an example of the application of the methods we present results related to polymorphic transitions for a classical potential model of iron.