Molecular oxygen tetramer (O₂)₄: Intermolecular interactions and implications for the ε solid phase

Recent data have determined that the structure of the high-pressure ε phase of solid oxygen consists of clusters composed of four O₂ molecules. This finding has opened the question about the nature of the intermolecular interactions within the molecular oxygen tetramer. We use multiconfigurational ab initio calculations to obtain an adequate characterization of the ground singlet state of (O₂)₄, which is compatible with the nonmagnetic character of the ε phase. In contrast to previous suggestions implying covalent bonding, we show that (O ₂)₄ is a van der Waals-like cluster where exchange interactions preferentially stabilize the singlet state. Nevertheless, as the cluster shrinks, there is an extra stabilization due to many-body interactions, i.e., an incipient chemical bonding that just yields a softening of the repulsive wall. We show that these findings can be used to model the intra- and intercluster distances of ε-O₂ observed in the lower-pressure range and are consistent with inelastic x-ray measurements of O₂ K-edge excitations.