Properties of the molecular oxygen trimer from pairwise additive interactions

We report a theoretical study of the properties of the molecular oxygen trimer in its state of maximum spin multiplicity. The most stable structures for the complex were found using an accurate pair potential. Independent ab initio calculations on the trimer for selected geometries demonstrate the validity of this approximation and thus the small contribution of three-body effects. The three lowest structures found can be rationalized on the basis of the most stable dimer structures and possess C ₂, D ₃ and C _2v point symmetries. The two lowest-lying structures are separated by only 0.6 meV and zero-point energy effects, which are sizable, reduce even further the gaps among the three structures studied. Diffusion Monte Carlo calculations indicate that the ground state vibrational wavefunction is mainly located around the C ₂ structure but also explores the D ₃ minimum, yielding a near oblate symmetric top with a binding energy of 32.1 meV.