Electron correlation effects on the structure of all 3d n4s m valence states of Ti, V, and Cr and their ions as studied by quasidegenerate many-body perturbation theory

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Abstract
The role of electron correlation on the electronic structure of the first row transition metal atoms Ti, V, and Cr and their ions is studied using the ab initio quasidegenerate many‐body perturbation theory based upon an effective valence shell Hamiltonian formalism. Second order Hv calculations yield good agreement with experiment for the excitation energies of all the 3dn4sm valence states of Ti, V, and their ions. The agreement is not as good for Cr and its ions. The contributions of various classes of excited configurations to the excitation energies of all valence states are studied in detail. The role of 3s and 3p core excitations and of excitations into the 4f virtual orbital are substantial in the second order Hv treatment. This is argued to be a general feature of transition metal electronic structure, and these effects are expected to be of similar importance in calculations employing other methods. Relativistic effects are estimated by comparison between relativistic and nonrelativistic self‐consistent field calculations and are found to be 0.1–0.4 eV for the neutral atoms, perhaps contributing substantially to discrepancies between theory and experiment.