Diatom-diatom interactions: Building potential energy surfaces and effect of intramolecular vibrations

An optimal procedure for obtaining spherically averaged potentials for the interaction between two linear centrosymmetric molecules (Koide and Kihara, Chem Phys 1974, 5, 34) is extended here for the calculation of the leading anisotropy term of the interaction. It is found that by computing the potential just at nine specific relative orientations of the monomers, the main anisotropy term of the spherical harmonic expansion can be calculated exactly provided that the expansion does not involve more than the first 14 terms. The approach also serves to investigate, in an affordable manner, the dependence of the interaction potential on the intramolecular degrees of freedom, and, in this way, to go beyond the commonly used rigid rotor approximation. Application has been performed for the [O₂(³Σ_g ^-)]₂ dimer by means of high-level ab initio calculations (RCCSD(T) method), and results are compared with a previous accurate rigid rotor potential energy surface. It is found that both the present isotropic and main anisotropy terms are very reliable and that the effect of the corrected vibrationally averaged potential is of about 1%. In addition, the dependence of the intermolecular potentials with the intramolecular vibrational modes is analyzed for the different geometries studied. The treatment can be quite useful in a realistic description of the main features of intermolecular interactions in more complex systems.