  Available simulation methods, suitable to describe solid-solid phase
transitions occurring upon increasing of presssure and/or temperature, are
based on empirical interatomic potentials: this restriction reduces the
predictive power, and thus the general usefulness of numeric simulations in
this very relevant field. We present a new simulation scheme which allows, for
the first time, the simulation of these phenomena with the correct
quantum-mechanical description of interatomic forces and internal stress, along
with the correct statistical mechanics of ionic degrees of freedom. The method
is obtained by efficiently combining the Car-Parrinello method for ab- initio
molecular dynamics with the Parrinello Rahman method to account for a variable
cell shape. Within this scheme phase trasformations may spontaneously take
place during the simulation with variation of external pressure and/or
temperature. The validity of the method is demonstrated by simulating the
metal-insulator transition in Silicon (from diamond structure to simple
hexagonal structure) under high pressure.
