HIGH-PRESSURE PHASES OF SIO2 USING LOCAL-DENSITY AND GENERALIZED-GRADIENT APPROXIMATIONS

Plane-wave calculations using the local-density approximation (LDA) ar e compared with those using the generalized-gradient approximation (GG A) in various high-pressure polymorphs of silica. The GGA gives lower cohesive energies and bulk modulii than the LDA but larger equilibrium unit-cell volumes. The cellular structure of the high-pressure post-s tishovite (CaCl2) phase is examined and suggested having a slightly la rger bulk modulus than stishovite. Trends in properties of the various phases are nearly similar using both approaches.