Evolution of the atomic structure and the magnetism of small oxygen clusters

Motivation for the present theoretical study has been provided by the experimental production of molecular oxygen clusters, and by the observation of an antiferromagnetic ordering of the localized molecular spins in the low temperature α-phase of solid oxygen. We have studied small (O2)n clusters with n = 2, 3 and 4, by performing unrestricted Møller–Plesset (UMP2) calculations. Focusing on magnetic states showing localized molecular spins, emphasis is placed on the analysis of the evolution of the magnetic behavior as the cluster size increases, and as the geometric arrangement of the oxygen molecules changes for fixed cluster size. Though the clusters treated are small, fingerprints of the cooperative antiferromagnetism known to occur in the low temperature phase of solid oxygen are already visible in the small clusters.