COMPUTER MODELLING OF PRESSURE-INDUCED PHASE TRANSFORMATIONS IN SOLIDS. John S. Tse and Dennis D. Klug, Steacie Institute for Molecular Sciences National Research Council of Canada Ottawa, Ontario, Canada K1A 0R6, Marco Bernasconi, Max-Planck Institut fur Festkorperforschung Postfach 80 06 65, D-70506 Stuutgart, Germany

The methods of Classical Molecular Dynamics (CMD) and First Principles total energy calculations have been demonstrated to be very valuable in the characterization of pressure-induced transformations in solids. However, each method when applied separately has specific limitations. CMD can be applied to systems with a large number of atoms but it suffers from the inherent difficulties associated with the uncertain accuracy of the potential functions when applied to conditions outside the regime of parameterization. In constrast, First Principles Molecular Dynamics (FPMD) can give highly accurate description of a system under any circumstances, the method is limited to relatively small systems with current computational facilities. The combination of the merits of both methods can lead to a very powerful procedure in the identification of structures at high pressures. In this procedure, possible high pressure crystalline forms are explored via large scale CMD calculations. The space group and internal coordinates of candidate structures are then extracted. FPMD is then used to optimize the unit cell and internal parameters. The diffraction patterns of the optimized structures can then be compared with experiments. This procedure will be illustrated through several examples on the high pressure structures of silica. The FPMD method has also been extended to include the Parrinello-Rahman variable cell scheme recently. The pressure effects on a structure can now be studied directly. New results on selected silica systems using this technique will be presented.