Theoretical studies of the N2O van der Waals dimer: Ab initio potential energy surface, intermolecular vibrations and rotational transition frequencies First-order (electrostatic and exchange) contributions to the O 2 el:g )-0 2 el:g ) 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 O 2 orientations and the intermolecular distance. The second-order polarization energy is evaluated through an analytical angular-dependent term for which the effective isotropic coefficient C 6 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 a phase of the solid oxygen. The structure of the van der Waals molecule (0 2 }z is discussed. Its energy is lowest for the parallel planar D2h geometry for the singlet (LlE min = -221 K at Re= 6.1 ao) and triplet (~Emin= -201 Kat R e =6.2 ao) states. The lowest energy for the quintet state (~Emin= -181 Kat R e =6.2 ao) is found for a crossed D2d structure. The (staggered) parallel and T-shaped structures are slightly higher in energy. The (02h is indeed a weakly bound molecule with hindered rotation around its van der Waals bond. The barrier for internal O 2 rotations around z (¢ angle) is estimated to be 50 K for the singlet, 27 K for the triplet, and 9 K for the quintet state.
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