Structure, energies, and vibrational properties of silica rings in glass

We have carried out ab initio molecular orbital calculations on four isomers of (Formula presented) modeling the local structure of (Formula presented) glass at the Hartree-Fock level. These clusters consist of two-, three-, four-, five-, and/or six-membered silica rings. The strain energies of the two-, three-, and four-membered rings are estimated by comparing the total energies of the relevant isomers. The strain energy of the four-membered ring is estimated to be 0.02 eV, indicating that the configuration of the four-membered ring is almost fully relaxed. The strain energies of the two- and three-membered rings are calculated to be 1.85 and 0.26 eV, respectively, and these values are in good agreement with the previous results calculated for the continuous (Formula presented) network models based on a generalized-gradient approximation to density-functional theory [D. R. Hamann, Phys. Rev. B 55, 14 784 (1997)]. It has been shown that there exist bonding wave functions that are localized in these small membered rings. We then have performed frequency calculations for the clusters, and the vibrational modes associated with these silica rings are discussed. © 2000 The American Physical Society.