It was recalled that recent  ab initio  determinations of the formation energy of an O vacancy were difficult to reconcile with thermodynamic data. It was shown here that theoretical calculations of the formation energy of an O vacancy yielded values that were larger than was expected on the basis of thermodynamic data. It was suggested that this was because the domain of non-stoichiometry of silica was much larger than suspected, because the interactions between O vacancies were very complex, or because the Hartree-Fock embedded cluster method and the MNDO-based methods tended to underestimate the formation energy of an O vacancy.

G.Boureau, S.Carniato: Solid State Communications, 1996, 98[6], 485-7