  Vibrational states of the formic acid molecule are converged using the
GENIUSH-Smolyak approach and the potential energy surface taken from [D. Tew
and W. Mizukami, J. Phys. Chem. A 120, 9815 (2016)]. The quantum nuclear motion
is described by using the $cis$-$trans$ torsional coordinate and eight
curvilinear normal coordinates defined with respect to an instantaneous
reference configuration changing as a function of the torsional degree of
freedom. Harmonic oscillator basis functions are used for the curvilinear
normal coordinates, a Fourier basis for the torsional coordinate, and a simple
basis pruning condition is combined with a Smolyak integration grid. $Trans$,
$cis$, and $delocalized$ vibrational states are reported up to and slightly
beyond the isomerization barrier.
