Electronic- and vibrational-structure calculations in models of the compressedSiO2glass system

The electronic and vibrational structures of normal and compressed ${\mathrm{SiO}}_2$ glasses are studied using realistic continuous-random-network models with periodic boundaries. The electronic structure is calculated by the first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method and the results are presented for density of states, band gaps, effective charges, and localization of wave functions as a function of density. The vibrational density of states, infrared absorption, and Raman spectra for these models are also calculated with use of a Keating-type potential and the bond-polarizability approximation. The results are, generally, in good agreement with experiments on both normal and ‘‘densified’’ a-${\mathrm{SiO}}_2$. In particular, the two Raman lines at 495 and 606 ${\mathrm{cm}}^{\mathrm{-}1}$ are reproduced by our calculation in direct contrast to the paracrystalline theory of ${\mathrm{SiO}}_2$ glass. Inspection of eigenvectors in conjunction with a detailed analysis of the bonding patterns of participating atoms indicate that specific Si-O-Si angles (close to those found in threefold or fourfold rings) are responsible for these modes.