Chemical Physics Letters vol:214 issue:6 pages:583-590
The effect of 3p correlation on the relative term energies in the (3d)n configurations of first-row transition metal ions has been studied using CASSCF and multireference perturbation theory. It is shown that (3p)2-->(3d)2 excitations contribute a significant part to the relative energies. Out of a total correlation contribution of -1.61 eV to the (3d)8(3F-->1S) excitation energy in Ni2+, -1.08 eV originates from the inter-shell double excitations. The corresponding numbers in Ti2+ are -1.16 and -0.37 eV, respectively. Similar but smaller effects are found for the neutral atoms.