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Intermolecular vibrations are extremely challenging to describe but are themost crucial part for determining entropy and hence free energies, and enablefor instance the distinction between different crystal-packing arrangements ofthe same molecule via THz spectroscopy. We introduce here a benchmark data set- V30 - containing 30 small molecular dimers with intermolecular interactionsranging from exclusively van-der-Waals dispersion to systems with hydrogenbonds. All calculations are performed with the gold standard of QuantumChemistry CCSD(T). We discuss vibrational frequencies obtained via differentmodels starting with the harmonic approximation over independent Morseoscillators up to second-order vibrational perturbation theory (VPT2), whichallows a proper anharmonic treatment including coupling of vibrational modes.However, the large amplitude motions present in many low-frequencyintermolecular modes are problematic for VPT2. In analogy to the often usedtreatment for internal rotations, we replace such problematic modes by a simpleone-dimensional hindered rotor model. We compare selected dimers with availableexperimental data or high-level calculations of potential energy surfaces andshow that VPT2 in combination with hindered rotors can yield a good descriptionof fundamental frequencies for the discussed subset of dimers involving smalland rigid molecules.

The V30 Benchmark Set for Anharmonic Vibrational Frequencies of Molecular Dimers