A van der Waals intermolecular potential for (O2)2

Abstract

First-order (electrostatic and exchange) contributions to the O2(3Σg−)–O2(3Σg−) interaction energy are computed ab initio and represented by a spherical expansion. The spin average energy as well as the Heisenberg exchange coupling parameter are fitted as a function of the O2 orientations and the intermolecular distance. The second-order polarization energy is evaluated through an analytical angular-dependent term for which the effective isotropic coefficient C6 is given by the treatment recently proposed by Cambi et al. for a generalized correlation in terms of polarizability. The resulting potential is in good agreement with the available experimental data for the gas phase (second virial coefficients) and for the ordered α phase of the solid oxygen. The structure of the van der Waals molecule (O2)2 is discussed. Its energy is lowest for the parallel planar D2h geometry for the singlet (ΔEmin=−221 K at Re=6.1 a0) and triplet (ΔEmin=−201 K at Re=6.2 a0) states. The lowest energy for the quintet state (ΔEmin=−181 K at Re=6.2 a0) is found for a crossed D2d structure. The (staggered) parallel and T-shaped structures are slightly higher in energy. The (O2)2 is indeed a weakly bound molecule with hindered rotation around its van der Waals bond. The barrier for internal O2 rotations around z (φ angle) is estimated to be 50 K for the singlet, 27 K for the triplet, and 9 K for the quintet state.