Theoretical studies of the bonding of oxygen to models of the (100) surface of nickel

Electronic wavefunctions have been obtained as a function of geometry for an O atom bonded to Ni clusters (consisting of one to five atoms) designed to model bonding to the (100) surface of Ni. Electron correlation effects were included using the generalized valence bond and configuration interaction methods. For the (100) surface, we find that the charge distribution for the full O overlay er is consistent with taking a positively charged cluster. The four surface atoms in the surface unit cell and the atom beneath the surface are important in determining the geometry, leading to a Ni⁺₅O cluster as the model for the (100) surface. The optimum oxygen position with this model is 0.96 Å above the surface (four-fold coordinate site) in good agreement with the value (0.90 ± 0.10 Å) from dynamic LEED intensity analysis. The atom beneath the surface allows important polarization effects for the positively charged cluster. The bonding to the surface involves bridging two diagonal surface Ni atoms. There is an O(2pπ) pair which overlaps the other diagonal pair of Ni atoms leading to nonbonded repulsions which increase the distance above the surface. There are two equivalent such structures, the resonance leading to a c(2 × 2) structure for the O overlayer.