Theoretical analysis of the bonding of oxygen to Cu(100)

The chemisorption of O/Cu(100) has been modeled by a Cu₅O cluster; ab initio self-consistent-field electronic wave functions have been obtained for this cluster. The bonding has been analyzed using several new theoretical methods: (1) the variation of the Cu₅O dipole moment with the distance of O from the surface; (2) the projection of the O orbitals from Cu₅O; and (3) the constrained space orbital variation (CSOV) method for the development of the bond. It is concluded that the bond is dominantly ionic but with a significant covalent contribution. Our results indicate that the excess charge on O is {~}1.5 electrons. We have computed the CSOV analysis for Cu₅O and for Cu₅ with point charges. The comparison of these two clusters has allowed us to have a definitive measure of the contribution of the Cu d electrons to the covalent bond. The total contribution of the d electrons to the bond is rather large (1.2 eV). Once the different contributions are separated, it is shown that purely electrical polarization effects account for {~}0.5 eV, while the direct participation of the d electrons in the covalent bond is {~}0.7 eV.