Anharmonic Vibrational Spectra of Molecular Dimers via Vibrational Perturbation Theory and Hindered Rotor Models

Accurate vibrational frequencies are crucial for the modeling of vibrational spectra and the calculation of free energies at finite temperatures. The anharmonic nature of individual normal modes as well as the coupling between modes can be captured by second-order vibrational perturbation theory (VPT2), which is mainly used to to describe (semi)-rigid isolated molecules. Herein, we assess the quality of VPT2 for intermolecular interactions utilizing a diverse set of small molecular dimers, which are held together by van der Waals dispersion interactions alone or also by intermolecular hydrogen bonds. Problematic large-amplitude motions like inter-/intramolecular rotations are described by hindered rotor models. First, we showcase the accuracy of this anharmonic approach by comparing our spectra obtained at the "gold standard" CCSD(T) level with available reference data. Then, we benchmark the quality of several density functionals in comparison to the CCSD(T) results. Later on, this approach can also be utilized for calculating anharmonic vibrational spectra of periodic molecular crystals via embedding methods.