Methane-water clusters under pressure: Are clathrate cages optimal clusters?

摘要

Global cluster structure optimization is applied to CH4u0001H2Ou0002n, n=4–21, in a simple model but withnthe additional application of external pressure. The combined effect of pressure and of the presencenof a methane molecule leads to a structural transition away from the well-known pure water clusternstructures. The exact location of this transition is a function of pressure and of cluster size, but itninvariably leads to “nestlike” structures as high-pressure form. Clathratelike cages around thenmethane molecule in the cluster center are less favorable than the best nests for all sizes and at allnpressures studied, but they do become low-energy local minima at high pressures.