Hartree-Fock calculations on linear NiCO and Ni2CO clusters have been used to model the local binding of CO upon adsorption on Ni. The Ni-CO bond is determined by a mixing of the CO(5σ) and Ni(3d) orbitals; the Ni(4s) electrons are not directly involved in the bond as can be seen from orbital contour plots. A knowledge of the absolute and relative shifts of molecular ionization potentials from their free-molecule values is important for the interpretation of the photoelectron spectra of chemisorbed species. For our model clusters, we have made a detailed analysis of the origins of the shifts for CO on Ni. The different contributions to the energy-level shifts of the CO-like orbitals show a grouping into core, nonbonding valence, and bonding valence levels. Chemical shifts due to the changed environment of the adsorbed molecule are seen to be important for the initial-state shifts of the nonbonding orbitals. A bonding shift appears to be important only for the CO 5σ level. The relaxation shifts are different among the different groups of levels. Analysis of the calculated and observed shifts gives support to the assignment that the order of the 5σ and 1π levels for CO adsorbed on Ni is the same as for free CO; 5σ less strongly bound than 1π. The O(1s) and C(1s) core-level shifts indicate that the CO bond is stretched only slightly upon adsorption. The multiplet splitting of the final ionic states is considered and found to be small. © 1977 The American Physical Society.