The O vacancy in silica was studied by using  ab initio  Hartree-Fock embedded cluster, and semi-empirical MNDO isolated cluster methods. It was found that the 2 approaches gave similar results in that the Si atoms near to the vacancy approached each other closely, the defect formation energies differed by about 1eV, and 2 localized defect states appeared in the fundamental gap: one fully occupied, and the other empty. A simplified configuration-interaction treatment, performed using the semi-empirical technique, gave an estimated value of 5eV for the lowest electron excitation energy.

V.B.Sulimov, C.Pisani, F.Corà, V.O.Sokolov: Solid State Communications, 1994, 90[8], 511-4