Pseudopotential periodic hartree-fock study of the cristobalite phases of silica and germanium dioxide

Periodic pseudopotential Hartree-Fock calculations have been performed on the cristobalite structure of silica and germanium dioxide in order to investigate the topology of the ground-state Born-Oppenheimer energy surface. For both oxides the averaged X-O bond lengths, the two unequivalent O-X-O bond angles, and the tilt angle have been optimized. The tilt angle, which measures the rotation of the XO 4 tetrahedra around their local C 2 axes, is directly related to the O-X-O bond angle and connects the low-temperature phase twin structures; its zero corresponds to the ideal β-phase