The defect energy which was associated with O-atom interstitials was calculated by using the Hartree-Fock linear combination of atomic orbitals method and the super-cell (or large unit cell) model. A neutral O-atom interstitial was added at volume centered, face-centered, or edge-centered positions. The largest super-cell which was used was chosen to contain 32 lattice atoms because previous studies, using the CNDO method, had demonstrated a satisfactory convergence of the defect energy, charge distribution, and defect levels in the band-gap, for super-cells of this size. It was found that, for all 3 defect positions, the most stable defect configuration was the symmetrical O--O- dumb-bell. This was formed by the displacement of one of the nearest-neighbor lattice ions, and was accompanied by a redistribution of charge such that the two O atoms in the dumb-bell each had a charge that was close to -1e. The most stable defect position was the face-centered dumb-bell.

P.W.M.Jacobs, R.A.Evarestov: Materials Science Forum, 1997, 239-241, 357-60