EXCITATION-ENERGIES IN THE NICKEL ATOM STUDIED WITH THE COMPLETE ACTIVE SPACE SCF METHOD AND 2ND-ORDER PERTURBATION-THEORY

The d8s2(3F), d9s(3D), d9s(1D), and d10(1S) states of the nickel atom have been studied using the complete active space (CAS) SCF method and a second order perturbation approach (CASPT2), which applies the CASSCF wavefunction as reference. A large basis set of the atomic natural orbital type was used. Three different active spaces were employed: (3d, 4s), (3d, 4s, 4p), and (3d, 4s, 4p, 3d'). The largest CASPT2 calculations correlated 18 electrons in 14 orbitals. These calculations yield the following excitation energies with respect to the 3D (d9s) ground state (relativistic corrections are included): 3F: -0.08 eV; 1D: 0.32 eV; 1S: 1.77 eV. The corresponding experimental excitation energies are (averaged over the j manifold): 0.03 (3F), 0.33 (1D), and 1.74 eV (1S), respectively. The following results are obtained when only the ten valence electrons are correlated: 3F: -0.19 eV; 1D: 0.30 eV; 1S: 1.90 eV and the results at the CASSCF level are: 3F: -0.76 eV; 1D: 0.32 eV; 1S: 2.17 eV.