High-pressure phases of SiO2 using local-density and generalized-gradient approximations

Plane-wave calculations using the local-density approximation (LDA) are compared with those using the generalized-gradient approximation (GGA) 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-stishovite (CaCl2) phase is examined and suggested having a slightly larger bulk modulus than stishovite. Trends in properties of the various phases are nearly similar using both approaches.