The semi-empirical method of intermediate neglect of differential overlap was used to make calculations for perfect and imperfect corundum crystals. In the case of the perfect crystal, the periodic large unit cell model was used. The molecular cluster model was used for defect calculations. The latter model was used to investigate the optical properties of electronic centers (F+, F, F-, FMg, FMg-) in corundum. The calculated optical properties of these defects were compared with the experimental values, and new bands were predicted to exist in the absorption spectrum of Mg-doped samples. The energy levels of F-type and Mg-related centers were found to lie in the gap between the upper valence band and the conduction band.

E.A.Kotomin, A.Stashans, P.W.M.Jacobs: Radiation Effects and Defects in Solids, 1995, 134, 87-90