The chemisorption of O/Cu(100) has been modeled by a Cu 5 O 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 Cu 5 O dipole moment with the distance of O from the surface; (2) the projection of the O orbitals from Cu 5 O; 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 Cu 5 O and for Cu 5 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.