Highly correlated systems. Ionization energies of first row transition metals Sc–Zn

The low-lying ionization potentials of the first row transition metal atoms Sc–Zn are calculated using fourth-order Mo/ller–Plesset perturbation theory (MP4) and quadratic configuration interaction (QCI) techniques with large spd and spdf basis sets. Two ionic states have been considered for each atom yielding a total of 20 different ionization processes which we have included in this study. For Sc+–Cu+, the ionic states considered have dns1 and dn+1 orbital occupations and for Zn+, the d10s1 and d9s2 states were studied. The MP4 method accurately reproduces the ionization potentials of Sc–Fe, but is found to be inadequate for Co–Zn. In contrast, the QCI technique performs uniformly for all ionization energies with a mean deviation from experiment of only 0.13 eV (with the spdf basis set) after inclusion of relativistic corrections.