Anharmonic Infrared Spectra of Thermally Excited Pyrene(C$_{16}$H$_{10}$): the combined view of DFT-based GVPT2 with AnharmonicCaOs and approximate DFT molecular dynamics with DemonNano

TL;DR

This paper compares two computational methods for modeling anharmonic infrared spectra of pyrene, aiming to improve the interpretation of astronomical data from the James Webb Space Telescope by balancing accuracy and efficiency.

Contribution

It introduces a benchmark comparison of quantum and molecular dynamics methods for anharmonic spectra and proposes a new empirical modeling approach for temperature-dependent IR emission.

Findings

AnharmoniCaOs is highly accurate but computationally expensive at high temperatures.

Molecular Dynamics with DFTB is faster but less precise, suitable for high-temperature modeling.

A new empirical recipe for anharmonic AIB emission modeling with minimal assumptions.

Abstract

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.