  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.
