  The study of the Aromatic Infrared Bands (AIBs) in astronomical environments
has opened interesting spectroscopic questions on the effect of anharmonicity
on the infrared (IR) spectrum of hot polycyclic aromatic hydrocarbons (PAHs)
and related species in isolated conditions. The forthcoming James Webb Space
Telescope will unveil unprecedented spatial and spectral details in the AIB
spectrum; significant advancement is thus necessary now to model the infrared
emission of PAHs, their presumed carriers, with enough detail to exploit the
information content of the AIBs. This requires including anharmonicity in such
models, and to do so systematically for all species included, requiring a
difficult compromise between accuracy and efficiency. We performed a benchmark
study to compare the performances of two methods in calculating anharmonic
spectra, comparing them to available experimental data. One is a full quantum
method, AnharmoniCaOs, relying on an ab initio potential, and the other relies
on Molecular Dynamics simulations using a Density Functional based Tight
Binding potential. The first one is found to be very accurate and detailed, but
it becomes computationally very expensive for increasing temperature; the
second is faster and can be used for arbitrarily high temperatures, but is less
accurate. Still, its results can be used to model the evolution with
temperature of isolated bands. We propose a new recipe to model anharmonic AIB
emission using minimal assumptions on the general behaviour of band positions
and widths with temperature, which can be defined by a small number of
empirical parameters. Modelling accuracy will depend critically on these
empirical parameters, allowing for an incremental improvement in model results,
as better estimates become gradually available.
