LIQUID METAL FREE ENERGIES FROM AB INITIO POTENTIAL SURFACES

Abstract

Direct free energy calculations for liquid metals from ab initio potential surfaces are very computationally intensive. Here we investigate Monte Carlo methods that involve sampling on the surface defined by a reference system. This allows for large gains in efficiency because the random walk is carried out on the (much faster) reference potential, and the ab initio potential is only evaluated on a small subset of uncorrelated configurations. We investigate the feasibility of these methods, by first applying them to liquid copper using an embedded atom potential, with a 1/r12 pair potential as the reference system. We find that the free energy perturbation series converges well and is numerically tractable. Second and third order perturbation theory give results that are weakly dependent on the reference system parameters. A preliminary application to an ab initio potential surface for liquid magnesium shows that we can obtain free energies accurate at the meV/atom level with only ∼100 evaluations of the ab initio total energies.