  Curvilinear kinetic energy models are developed for variational nuclear
motion computations including the inter- and the low-frequency intra-molecular
degrees of freedom of the formic acid dimer. The coupling of the inter- and
intra-molecular modes is studied by solving the vibrational Schr\"odinger
equation for a series of vibrational models, from two up to ten active
vibrational degrees of freedom by selecting various combinations of active
modes and constrained coordinate values. Vibrational states, nodal assignment,
and infrared vibrational intensity information is computed using the the
full-dimensional potential energy surface (PES) and electric dipole moment
surface developed by Qu and Bowman [Phys. Chem. Chem. Phys. 18, 24835 (2016);
J. Chem. Phys. 148, 241713 (2018)]. Good results are obtained for several
fundamental and combination bands in comparison with with jet-cooled
vibrational spectroscopy experiments, but the description of the $\nu_8$ and
$\nu_9$ fundamental vibrations, which are close in energy and have the same
symmetry, appears to be problematic. For further progress in comparison with
experiment, the potential energy surface, and in particular, its
multi-dimensional couplings representation, requires further improvement.
