Rovibrational computations for molecular dimers
Fingerprint vibrations of the formic acid dimer, including all fundamental and several combination and overtone bands, have been reported with an experimental uncertainty of 1 cm-1 , while computational results deviate from experiment by 10‒50 cm-1. To clarify the origin of the large differences of theory and experiment, we have carried out  variational vibrational computations using curvilinear and normal coordinate vibrational models from 2 up to 10 fully coupled vibrational degrees of freedom using the GENIUSH computer program  and the full-dimensional potential energy surface (PES) developed by Qu and Bowman . A good agreement is achieved with experiment for several fingerprint vibrational bands, but we have found indications for a necessary improvement of the PES before further vibrational computations (of higher dimensionality and/or including the tunneling mode) can be carried out.
Regarding the description of coupled large-amplitude motions, we elaborate on the convergence of rovibrational energies for systems where the amplitude of the wave function is significant in the singular region . For this purpose, we benchmark the numerical convergence and computational efficiency of the black-box-type rovibrational procedure of GENIUSH with respect to a tailor-made dimer Hamiltonian approach  for the example of the intermolecular dynamics of the methane-water dimer.
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