Using the unrestricted Hartree–Fock method and super-cells containing up to 160 atoms, calculations were made of the energy level positions in the gap and atomic geometry for the Fe4+ impurity substituting for a host Ti atom in SrTiO3. In agreement with experiment, the high-spin (S = 2) state was much lower in energy than the zero-spin state. The energy level positions depended strongly upon the asymmetric displacement mode of the 6 nearest O ions which was a combination of the Jahn–Teller and breathing modes. Considerable covalent bonding between the Fe ion and the 4 nearest O ions took place.

Large-Scale ab initio Modelling of Defects in Perovskites - Fe Impurity in SrTiO3. E.A.Kotomin, R.A.Evarestov: Computational Materials Science, 2002, 24[1-2], 14-20