  By suitably adapting a recent approach [A. Laio and M. Parrinello, PNAS, 99,
12562 (2002)] we develop a powerful molecular dynamics method for the study of
pressure-induced structural transformations. We use the edges of the simulation
cell as collective variables. In the space of these variables we define a
metadynamics that drives the system away from the local minimum towards a new
crystal structure. In contrast to the Parrinello-Rahman method our approach
shows no hysteresis and crystal structure transformations can occur at the
equilibrium pressure. We illustrate the power of the method by studying the
pressure-induced diamond to simple hexagonal phase transition in a model of
silicon.
