A theory for molecular collision cross section due to quadrupole-induced dipole and dipole-induced quadrupole interactions has been developed following the Anderson—Tsao—Curnutte approach. The OCS–He, OCS–Ar, and HCl–Ar collisions are discussed. For HCl–Ar collisions, μ1q1α2 interactions have been found to be important, whereas in OCS–He and OCS–Ar collisions these are negligibly small.
Describing intermolecular forces is fundamental to modeling and predicting the behavior of molecular systems. In particular, long-range molecular interactions─with electrostatic, induction, and dispersion as the main components─play a critical role, especially for low-temperature and low-density regimes. Long-range interactions are often described through perturbation theory, representing the electronic charge distribution via a multipolar series of the moments and polarizability tensors corresponding to each molecule. However, while the theory is well established, obtaining the resulting analytical expressions (and their practical implementation) constitutes a highly complex and system-dependent task. To address this challenge, we developed long-range-fit (LRF), an interactive and user-friendly software package designed to automate the generation and fitting of long-range interaction terms for arbitrary molecules in nondegenerate (ground or excited) electronic states. We have derived and implemented all terms up to 15th order, without approximations, via a spherical tensor representation, with symmetry adaptation to all molecular point-group symmetries. The resulting potential energy surface is compatible with most representations of the close interaction region.
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