Valence correlation in the s2d n, s d n+1, and d n+2 states of the first-row transition metal atoms

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Abstract
The major differential valence correlation effects of the lowest lying states arising from the s2dn, sdn+1, and dn+2 configurations of the first‐row transition metal atoms have been characterized using MCSCF and CI procedures. The important correlation effects are found to be, first, angular correlation of the 4s2 pair arising because of the near degeneracy of the 4s and 4p orbitals and, second, radial correlation of the 3d electron pairs. This large differential radial correlation of the 3d electrons can be interpreted as being due to nonequivalent d orbitals in the sdn+1 and dn+2 excited states. Both of these effects can be incorporated into a simple MCSCF wave function that reduces the error in the excited state atomic dissociation limits (∼0.2 eV in Sc–Cr and ∼0.5 eV in Mn–Cu for the sdn+1–s2dn excitation energy), yet still is of a form which lends itself easily to molecular calculations.