PRESSURE-INDUCED PHASE-TRANSITIONS IN SOLID SI, SIO2, AND FE - PERFORMANCE OF LOCAL-SPIN-DENSITY AND GENERALIZED-GRADIENT-APPROXIMATION DENSITY FUNCTIONALS

We focus on a particular shortcoming of the local-spin-density (LSD) a pproximation for the exchange-correlation energy of a many-electron sy stem: underestimation of the transition pressure p(t) at pressure-indu ced structural phase transitions in solids. We have performed self-con sistent full-potential LAPW calculations, with full structure optimiza tion, for three cases-silicon (Si), silica (SiO2), and iron (Fe). In a greement with previous calculations, we fmd that gradient corrections to LSD over-correct the equilibrium volumes in Si and SiO2, but correc t or slightly under-correct the volume in Fe and the transition pressu res in all three materials. We apply a thermodynamiclike inequality [A . Zupan et at, J. Chem. Phys. 106, 10 184 (1997)] to our results to ex plain why the generalized gradient approximation expands the equilibri um volume and increases the value of p(t) over LSD. In all three cases , gradient corrections to LSD tend to stabilize the low-pressure phase because of its more inhomogeneous electron density. [S0163-1829(98)05 241-2].