Ab initio self-consistent-field molecular orbital calculations on defects associated with radiation damage in alpha quartz

A series of ab initio self-consistent-field molecular orbital (SCF-MO) calculations, on defect centers involved in radiation damage of α quartz, is reported. Calculations were performed on a 21 atom cluster (Si ₅O₁₆^12-), using a 3-21 G basis set. The long-range Coulomb interactions of a quartz were included, by surrounding the 21 atom cluster with 956 point charges, so that on geometry optimization, the characteristic features of α quartz were maintained. Various defect centers containing A1³⁺ substitutional ions, and their compensating electron-hole and proton defects were studied. The results obtained for the [A1O₄]⁰ center are in close agreement with electron spin resonance (ESR) data. Calculations were also performed on the oxygen vacancy and related defects, such as the E ′₁ and E ′₄ defects, where qualitative agreement with ESR results was obtained. An assessment is made, of the effect of including long-range Coulomb interactions, by comparison with other SCF-MO cluster models of these systems, which do not include these electrostatic effects.