Interaction of atomic oxygen with a Cu5 (Ag5) cluster model simulating the fourfold chemisorption of oxygen on Cu(100) and Ag(100) has been studied at the self-consistent-field (SCF) and configuration interaction (CI) levels using one-electron pseudopotentials to describe the inner shells of the cluster metal atoms. Core-polarization effects are introduced by means of a second-order perturbation method. Results show the method to be reliable and that there is no need for any molecular adjustment of the pseudopotential. Nondynamical and dynamical valence correlation effects are found to be very important. The separability of valence correlation effects is analyzed by comparison with previous calculations explicitly including the d electrons of the cluster metal atoms. Core-valence correlation effects decrease the interaction energy calculated at the valence CI level and are shown to be very important not only from the quantitative point of view but also from the qualitative one because different electronic states are differently affected, leading to a change in the ordering of the electronic states with respect to that found at the CI valence calculation level. © 1990 American Institute of Physics.