Generalized Valence Bond Description of the Low-Lying States of NiCO

GVB and GVB-CI wave functions (using a double ζ basis) have been obtained as a function of internuclear distance for the lowest three states of NiCO. The wave functions lead to a qualitative description in which the Ni atom is neutral with a (4s)¹(3d)⁹ atomic configuration. The CO lone pair delocalizes slightly onto the Ni leading to the 4s-like orbital hybridizing away from the CO. The dπ pairs on the Ni are slightly back-bounding to the CO. The three bound states are ³Σ⁺, ³II, and ³Δ consisting of the singly occupied 4s-like orbital plus a single d hole in a σ, π, or δ orbital, respectively. The ground state is found to be ³Δ with calculated R_e = 1.90 Å, D_e = 1.15 eV = 26.5 kcal/mol, and ω_e(Ni-C) = 428 cm^-1, all reasonable values, although direct information on NiCO is not yet available. The adiabatic excitation energies are calculated as 0.240 eV to ³Σ⁺ and 0.293 eV to ³II. The states with (4s)²(3d)⁸ configurations on the Ni lead to repulsive potential curves with vertical excitation energies in the range of 3.0 to 5.0 eV.