Nonequilibrium free-energy calculation of phase-boundaries using LAMMPS

TL;DR

This paper introduces a non-equilibrium free-energy calculation method within LAMMPS for accurately determining phase boundaries across wide thermodynamic ranges, demonstrated on silicon's phase diagram.

Contribution

It presents the dCCI method for efficient phase boundary computation and provides implementation details and source code within LAMMPS.

Findings

Successfully computed silicon's phase diagram from 0 to 15 GPa and 400 K.

Determined phase boundaries and triple point between diamond, liquid, and beta-Sn phases.

Demonstrated the effectiveness of non-equilibrium methods for wide-range free-energy calculations.

Abstract

We present a guide to compute the phase-boundaries of classical systems using a dynamic Clausius-Clapeyron integration (dCCI) method within the LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) code. The advantage of the dCCI method is because it provides coexistence curves spanning a wide range of thermodynamic states using relatively short single non-equilibrium simulations. We describe the state-of-the-art of non-equilibrium free-energy methods that allow us to compute the Gibbs free-energy in a wide interval of pressure and/or temperature. We present the dCCI method in details, discuss its implementation in the LAMMPS package and make available source code, scripts, as well as auxiliary files. As an illustrative example, we determine the phase diagram of silicon in a range of pressures covering from 0 to 15 GPa and temperatures as low as 400 K up to the liquid phase, in order to obtain the phase boundaries and triple point between diamond, liquid and beta-Sn phases.