First-principles study of cubic perovskites SrMO_3 (M = Ti, V, Zr and   Nb)
  Using the full-potential linearized-augmented-plane-wave (FLAPW) method, we have analyzed systematically the trends in the structural and electronic properties of the 3d and 4d transition-metal oxides SrMO_3 (M = Ti, V, Zr and Nb). The optimized lattice parameters, bulk modules, densities of states, band structures and charge density distributions are obtained and compared with the available theoretical and experimental data. The energy gap between O2p - Md bands increases as the covalency of the system decreases going from 3d to 4d based perovskites. The electron configurations of Sr(Ti,Zr)O_3 and Sr(V,Nb)O_3 usually referred to as d^0 and d^1 oxides, respectively, differ considerably from these idealized "ionic" configurations, and the deviations increase with increasing of the d-p covalent overlap in the oxides. 
I.R. Shein, V.L.Kozhevnikov and A.L. Ivanovskii