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 in 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 ∼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 β-tin phase.