  Phonons crucially impact a variety of properties of organic semiconductor
materials. For instance, charge- and heat transport depend on low-frequency
phonons, while for other properties, such as the free energy, especially
high-frequency phonons count. For all these quantities one needs to know the
entire phonon band structure, whose simulation becomes exceedingly expensive
for more complex systems when using methods like dispersion-corrected density
functional theory (DFT). Therefore, in the present contribution we evaluate the
performance of more approximate methodologies, including density functional
tight binding (DFTB) and a pool of force fields (FF) of varying complexity and
sophistication. Beyond merely comparing phonon band structures, we also
critically evaluate to what extent derived quantities, like
temperature-dependent heat capacities, mean squared thermal displacements and
temperature-dependent free energies are impacted by shortcomings in the
description of the phonon bands. As a benchmark system, we choose (deuterated)
naphthalene, as the only organic semiconductor material for which to date
experimental phonon band structures are available in the literature. Overall,
the best performance amongst the approximate methodologies is observed for a
system-specifically parametrized second-generation force field. Interestingly,
in the low-frequency regime also force fields with a rather simplistic model
for the bonding interactions (like the General Amber Force Field) perform
rather well. As far as the tested DFTB parametrization is concerned, we obtain
a significant underestimation of the unit cell volume resulting in a pronounced
overestimation of the phonon energies in the low frequency region. This cannot
be mended by relying on the DFT-calculated unit cell, since with this unit cell
the DFTB phonon frequencies significantly underestimate the experiments.
