The electronic structure of NiCO: A new prediction for the ground state

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Abstract
We have performed electronic structure calculations on the NiCO molecular fragment through the configuration interaction level. Our initial motivation was to compare the bonding in the low lying triplet states 3Δ, 3Σ+, and 3Π with that in the lowest closed shell singlet state 1Σ+. In both the σ and the π systems the Ni–CO bond is stronger in the 1Σ+ state than in the others. The σ-bonding difference is a consequence of the occupation of the strongly antibonding Ni 4s-like orbital in the triplet states, whereas in the 1Σ+ state it is unoccupied. The π-bonding difference arises from the greater π-backbonding ability of the more diffuse Ni 3d orbitals present in the 1Σ+ state. Most notably, though, we contradict previously published results by predicting a 1Σ+ ground state rather than a 3Δ ground state. A many configuration wave function is essential in accurately describing the π backbonding as well as producing the correct atomic energy level splittings. It is, for the most part, these two factors which produce the great stabilization of the 1Σ+ state with respect to the triplet states on adding configuration interaction to the Hartree–Fock result.