Methane-water clusters under pressure: are clathrate cages optimal clusters?
Global cluster structure optimization is applied to CH(4)(H(2)O)(n), n=4-21, in a simple model but with the additional application of external pressure. The combined effect of pressure and of the presence of a methane molecule leads to a structural transition away from the well-known pure water cluster structures. The exact location of this transition is a function of pressure and of cluster size, but it invariably leads to "nestlike" structures as high-pressure form. Clathratelike cages around the methane molecule in the cluster center are less favorable than the best nests for all sizes and at all pressures studied, but they do become low-energy local minima at high pressures.
Hartke
Bernd
B
Institut fur Physikalische Chemie, Christian-Albrechts-Universitat, Olshausenstrasse 40, 24098 Kiel, Germany. hartke@phc.uni-kiel.de