Geometry, vibrational frequencies, and ionization potentials for CO/Ni(100); explanation of the disappearance of the 5σ peak in pes

Using a cluster of 14 Ni atoms to model a Ni(100) surface, we used ab initio methods [generalized valence bond (GVB)] to study CO chemisorbed at the on-top site. Reported are the optimum geometry, vibrational frequencies, and ionization potentials. We propose a new explanation for the two lowest levels of free CO (5σ and 1π) reducing to one level of chemisorbed CO.