The geometrical and physical model for adsorption of oxygen on silicon has not been well established. In the literature proposed models have included atomic oxygen, different surface sites and bond multiplicities, and molecular oxygen. We have performed calculations using an ab initio multiconfiguration SCF cluster approach as well as a slab simulation in the framework of an SCF pseudopotential approach. From the results of the cluster and slab calculations we find that oxygen in the first layer bridging positions is more stable than in the on-top configuration. Further, the fourfold center site is shown to be quite weakly bound and thus unlikely to be a stable adsorption site. For O(1×1)—Si(100) the equilibrium bond lengths in the on-top and bridging sites are, from the slab calculations, found to be 1.76 and 2.15 A. The corresponding values obtained from our cluster calculations are 1.64 and 1.92 A. We report vibrational frequencies for various sites and suggest a possible interpretation of the EELS data in terms of an atomic chemisorption model. © 1984, American Vacuum Society. All rights reserved.