The atomic states of nickel

The Ni 3d⁹ 4s¹(³D)–3d¹⁰(¹S) and 3d⁹ 4s¹(³D)-3d⁸4s²(³F) atomic excitation energies have been computed using large multireference CI wave functions in conjunction with a large ANO basis set. Radial correlation effects in the 3d shell are found to be very important and are included using CASSCF wave functions having the 3d and correlating 3d orbitals in the active space. The previous discrepancy (0.5 eV) with experiment for the ³D–¹S excitation is reduced to 0.1 eV when the 3d3d references are included in the CI. For the ³F state, the 4s-4p near degeneracy gives rise to important 4s²4p² excitations in addition to the 3d3d excitations which are important for the ³D and ¹S states. Inclusion of only 3d-3d correlation in the ³F and ³D CI reference spaces yields a ³F–³D separation which is in error by 0.12 eV. Addition of the 4s²4p² excitations to the ³F reference space is estimated to increase the discrepancy with experiment by an additional 0.1 eV.