Molecular orbital correlation diagrams for He2, He2+, N2, N2+, CO, and CO+

Abstract

After a preliminary check with He₂ and He₂⁺, self‐consistent field calculations have been carried out for the nitrogen and carbon monoxide molecules and some of their positive ions for the range of internuclear distances from about 1.5 times equilibrium down to 0.01 bohr. To adequately describe the passage from the separated to the united atom limits, basis sets comprised of even‐tempered expansions of Slater type s and p functions were centered at each nucleus and at a point midway between them. This three center procedure is compared with that of reoptimizing basis functions at each internuclear distance. It is found that the three‐center approach almost completely obviates the need of the polarization functions which must be added if a two‐center approach is used. The correlation diagrams for nitrogen and carbon monoxide and the first positive ions of each symmetry type are all similar in shape, especially for the innermost orbitals. Ionization potential curves plotted on the diagrams for the neutral systems are nearly parallel to the corresponding orbital energy curves. This indicates that Koopmans’ theorem is consistent at all distances and implies that correlation diagrams for neutral molecules should be reasonable approximations to the ionization potential curves. For the most easily ionized orbital of both N₂ and especially CO, the ionization potential has a remarkably low value at about one‐third the equilibrium separation. To adequately describe this orbital an extra s function having a smaller zeta than the smallest one in the united atom basis set was required. This implies that merely using the optimized basis sets for the separated and united atoms in a three‐center procedure is scarcely acceptable in the region where the outermost orbital energy has the lowest value.