FIRST PRINCIPLES CALCULATIONS OF CRYSTAL STRUCTURES AND TRANSITIONS UNDER HIGH PRESSURE. Andres Mujica(1), Richard .J. Needs(2), and Alfonso Munoz(1), (1)Dpto. de Fisica Fundamental y Experimental, Universidad de La Laguna, La Laguna E-38205, Tenerife, Spain, (2) Theory of Condensed Matter Group, Cavendish Laboratory, Cambridge CB3 0HE, United Kingdom

In this talk we review the current state-of-the-art in the theoretical study of crystal structures, based on the Density Functional Theory (DFT). The detail and precision now achievable will be illustrated with recent results from a DFT study of the structural stability of several III-V and II-VI binary compounds under high pressure, with an emphasis on the recently discovered Cmcm phase. Experimental results and our ab initio calculations are in excellent agreement, and establish the stability of a Cmcm structure in a variety of compounds. We have investigated the mechanism leading to the Cmcm phase and the evolution of its structural parameters with increasing pressure. We also show how Cmcm can be related to previously suggested structures, and thus how previous experimental observations can be reconciled within the present status of knowledge.