Solids under pressure. Ab initio theory

Parameter-free calculations based on the density-functional theory are used to examine high-pressure phases of solids, mainly semiconductors. For the elemental semiconductors, as represented by Si, the diamond --> beta-tin--> Imma sequence is examined, and for III-V semiconductors the optimization of the structural parameters of the Cmcm and Imm2 phases is described. The st ructural energy differences are in several cases very small, and in some to o small to allow a safe structure prediction on the basis of the calculatio ns. In that context we also discuss ways to go beyond the local density app roximation (LDA). We show that the predicted high-pressure phases may be si gnificantly affected by inclusion of (generalized) gradient corrections (GG A). Elemental Zn (h.c.p.) is further taken as an example where we find that the simple LDA leads to poor results.