A new constant-pressure ab initio/classical molecular dynamics method:: simulation of pressure-induced amorphization in a Si35H36 cluster

We present here a new constant-pressure ab initio molecular dynamics method, suitable, e.g., for studying pressure-induced structural transformations ill finite non-periodic systems such as clusters. In order to apply external isotropic pressure on the cluster, we immerse an ab initio treated cluster in a model classical liquid, described by a repulsive soft-sphere potential, which acts as a pressure reservoir. The extended system cluster + liquid is simulated by a coupled Car-Parrinello and classical molecular dynamics. The pressure is varied by tuning the parameter of the liquid potential. We apply the method to a Si35H36 cluster, which undergoes a pressure-induced amorphization at similar to 35 GPa, and remains in a disordered state even upon pressure release. The properties of cluster at different pressures are analyzed by means of maximally localized Wannier functions method. In the high-pressure phase, a considerable reduction of the Kohn-Sham energy gap as well as an increase of electronic delocalization is observed, which represents an analogue of metallization of bulk Si upon transition from diamond to beta -tin phase. (C) 2001 Elsevier Science B.V. All rights reserved.