  While powerful techniques exist to accurately account for anharmonicity in
vibrational molecular spectroscopy, they are computationally very expensive and
cannot be routinely employed for large species and/or at non- zero vibrational
temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH)
emission in space, we developed a new code, which takes into account all modes
and can describe all IR transitions including bands becoming active due to
resonances as well as overtones, combination and difference bands. In this
article, we describe the methodology that was implemented and discuss how the
main difficulties were overcome, so as to keep the problem tractable.
Benchmarking with high-level calculations was performed on a small molecule. We
carried out specific convergence tests on two prototypical PAHs, pyrene
(C$_{16}$H$_{10}$) and coronene (C$_{24}$H$_{12}$), aiming at optimising
tunable parameters to achieve both acceptable accuracy and computational costs
for this class of molecules. We then report the results obtained at 0 K for
pyrene and coronene, comparing the calculated spectra with available
experimental data. The theoretical band positions were found to be
significantly improved compared to harmonic Density Functional Theory (DFT)
calculations. The band intensities are in reasonable agreement with
experiments, the main limitation being the accuracy of the underlying
calculations of the quartic force field. This is a first step towards
calculating moderately high-temperature spectra of PAHs and other similarly
rigid molecules using Monte Carlo sampling.
