  Intermolecular vibrations are extremely challenging to describe but are the
most crucial part for determining entropy and hence free energies, and enable
for instance the distinction between different crystal-packing arrangements of
the same molecule via THz spectroscopy. We introduce here a benchmark data set
- V30 - containing 30 small molecular dimers with intermolecular interactions
ranging from exclusively van-der-Waals dispersion to systems with hydrogen
bonds. All calculations are performed with the gold standard of Quantum
Chemistry CCSD(T). We discuss vibrational frequencies obtained via different
models starting with the harmonic approximation over independent Morse
oscillators up to second-order vibrational perturbation theory (VPT2), which
allows a proper anharmonic treatment including coupling of vibrational modes.
However, the large amplitude motions present in many low-frequency
intermolecular modes are problematic for VPT2. In analogy to the often used
treatment for internal rotations, we replace such problematic modes by a simple
one-dimensional hindered rotor model. We compare selected dimers with available
experimental data or high-level calculations of potential energy surfaces and
show that VPT2 in combination with hindered rotors can yield a good description
of fundamental frequencies for the discussed subset of dimers involving small
and rigid molecules.
