Lattice-switch Monte Carlo

TL;DR

This paper introduces a lattice-switch Monte Carlo method for directly calculating free energy differences between crystal structures, enabling efficient sampling and precise comparison of structures like fcc and hcp.

Contribution

The paper presents a novel lattice-switch Monte Carlo technique that allows direct free energy difference evaluation between crystal phases using biased sampling.

Findings

Efficiently samples configurations to compare free energies of crystal structures.

Shows high-precision free energy differences between fcc and hcp.

Demonstrates the method's effectiveness in constant density and pressure ensembles.

Abstract

We present a Monte Carlo method for the direct evaluation of the difference between the free energies of two crystal structures. The method is built on a lattice-switch transformation that maps a configuration of one structure onto a candidate configuration of the other by `switching' one set of lattice vectors for the other, while keeping the displacements with respect to the lattice sites constant. The sampling of the displacement configurations is biased, multicanonically, to favor paths leading to `gateway' arrangements for which the Monte Carlo switch to the candidate configuration will be accepted. The configurations of both structures can then be efficiently sampled in a single process, and the difference between their free energies evaluated from their measured probabilities. We explore and exploit the method in the context of extensive studies of systems of hard spheres. We show that the efficiency of the method is controlled by the extent to which the switch conserves correlated microstructure. We also show how, microscopically, the procedure works: the system finds gateway arrangements which fulfill the sampling bias intelligently. We establish, with high precision, the differences between the free energies of the two close packed structures (fcc and hcp) in both the constant density and the constant pressure ensembles.