1 Periodic LUC study of F centers in cubic and tetragonal SrTiO 3 Arvids Stashans*, Fernando Vargas Centro de Investigación en Física de la Materia Condensada, Corporación de Física Fundamental y Aplicada, Apartado 17-12-637, Quito, Ecuador Abstract Modified for crystals quantum-chemical computational approach and the periodic LUC (Large Unit Cell) model is used for calculating structural, electronic and optical properties of F centers (two electrons trapped by an oxygen vacancy). The automated geometry optimization is carried out for the ground state of defect in both cubic and tetragonal crystalline lattices of SrTiO 3 . The wave functions of two extra electrons extend over the two vacancy-closest Ti atoms and over other nearby atoms. The absorption energies are calculated using the ∆SCF scheme after the performance of system’s geometry optimization. The obtained results are discussed in a comparative manner for two different crystallographic phases. PACS: 71.15.Fv; 77.84.Dy Keywords: Perovskites; Strontium titanate; F centers; Quantum-chemical calculations. ______________ * Corresponding author. FAX: +593-2-403656. E-mail address: sauleskalns@yahoo.com (A. Stashans).