A theoretical study of the dissociation energy of Ni-2(+) a case of broken symmetry

The electronic structure and potential curves for the lowest states of the Ni-2(+) 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 a-hole states break symmetry at all internuclear distances longer than equilibrium. CASSCF and CASPT2 calculations were first performed using C-2v symmetry. Full symmetry was restored by mixing two symmetry related CASSCF wavefunctions. Ni-2(+) has a (4) Sigma(u)(-) ground state with a computed bond energy of 2.47 eV and a bond distance of 2.19 Angstrom. 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 D-2h symmetry give a binding energy (D-0) of 2.25 eV and a bond distance of 2.22 Angstrom, in apparent agreement with experiment (D-0 = 2.25 eV, r(e) = 2.22 Angstrom), indicating that the symmetry breaking error is independent of the internuclear distance.