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. © 1990 The American Physical Society.