We have used the ab initio technique of UHF/LANL 1 MB to study the interactions between absorbed oxygen atoms and molecules with their host nickel lattice. The host lattice is approximated by a small cluster of nickel atoms that are arranged as face-centered-cubic, except for a tubular void described by the absence of three or four lattice elements. The oxygen absorbates are placed, one at time, within this cavity. We conclude that, for tubular cavities, the interaction between the nickel lattice and the oxygen absorbates is stronger than the interactions between pairs of oxygen atoms, suggesting that the absorbates are mostly atomic (as apposed to diatomic) in character, while the (energetically) most preferred number of oxygen atoms to reside inside an n-site, tubular hollow of nickel is simply (n − 1). © 1993 John Wiley & Sons, Inc.