The quantum-chemical computational approach and the periodic large unit cell model, as modified for crystals, were used to calculate the structural, electronic and optical properties of F centers (2 electrons trapped by an O vacancy). Geometrical optimization was carried out for the ground state of the defect in both the cubic and tetragonal lattices. The wave functions of 2 extra electrons were found to extend over the 2 vacancy-adjacent Ti atoms and over other nearby atoms.

Periodic LUC Study of F Centers in Cubic and Tetragonal SrTiO3. A.Stashans, F.Vargas: Materials Letters, 2001, 50[2-3], 145-8