A theoretical study of the dissociation energy of Ni2+ A case of broken symmetry

The electronic structure and potential curves for the lowest states of the Ni2+ cation have been studied using multiconfigurational SCF theory (CASSCF) combined with second-order perturbation theory (CASPT2) and non-orthogonal CI. The wavefunctions for the so-called σ-hole states break symmetry at all internuclear distances longer than equilibrium. CASSCF and CASPT2 calculations were first performed using C2v symmetry. Full symmetry was restored by mixing two symmetry related CASSCF wavefunctions. Ni2+ has a 4Σu− ground state with a computed bond energy of 2.47 eV and a bond distance of 2.19 Å. The approach leads to a somewhat large binding energy due to double counting of the resonance energy. An estimate of the error yields a binding energy of slightly less than 2.4 eV. Calculations using D2h symmetry give a binding energy (D0) of 2.25 eV and a bond distance of 2.22 Å, in apparent agreement with experiment (D0 = 2.25 eV, re = 2.22 Å), indicating that the symmetry breaking error is independent of the internuclear distance.