Highly correlated systems. Excitation energies of first row transition metals Sc–Cu

The low-lying dns2→dn+1s1 excitation energies of the first row transition metal atoms Sc–Cu are calculated using fourth-order M≂ller–Plesset perturbation theory (MP4) as well as quadratic configuration interaction (QCI) techniques with large spd and spdf basis sets. The MP4 method performs well for Sc–Mn but fails dramatically for Fe–Cu. In contrast, the QCI technique performs uniformly for all excitation energies with a mean deviation from experiment of only 0.14 eV after including relativistic corrections. f functions contribute 0.1–0.4 eV to the excitation energies for these systems. The highly correlated d10 state of the Ni atom is also considered in detail. The QCI technique obtains the d9s1→d10 splitting of the Ni atom with an error of only 0.13 eV. The results show that single-configuration Hartree–Fock based methods can be successful in calculating excitation energies of transition metal atoms.