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) approximation for the exchange-correlation energy of a many-electron system: underestimation of the transition pressure p(t) at pressure-induced structural phase transitions in solids. We have performed self-consistent full-potential LAPW calculations, with full structure optimization, for three cases-silicon (Si), silica (SiO2), and iron (Fe). In agreement with previous calculations, we fmd that gradient corrections to LSD over-correct the equilibrium volumes in Si and SiO2, but correct or slightly under-correct the volume in Fe and the transition pressures in all three materials. We apply a thermodynamiclike inequality [A. Zupan et at, J. Chem. Phys. 106, 10 184 (1997)] to our results to explain why the generalized gradient approximation expands the equilibrium 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)05241-2].