Determining surface phase diagrams including anharmonic effects

We introduce a massively parallel replica-exchange grand-canonical sampling algorithm to simulate materials at realistic conditions, in particular surfaces and clusters in reactive atmospheres. Its purpose is to determine in an automated fashion equilibrium phase diagrams for a given potential-energy surface and for any observable sampled in the grand-canonical ensemble. The approach enables an unbiased sampling of the phase space and is embarrassingly parallel. It is demonstrated for a model of the Lennard-Jones system describing a surface in contact with a gas phase. Furthermore, the algorithm is applied to SiM clusters (M = 2, 4) in contact with an H-2 atmosphere, with all interactions described at the ab initio level, i.e., via density-functional theory, with the Perdew-Burke-Ernzerhof gradient-corrected exchange-correlation functional. We identify the most thermodynamically stable phases at finite T, p(H-2) conditions.