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 infrared transitions including bands becoming active due to resonances as well as overtone, 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 (C₁₆H₁₀) and coronene (C₂₄H₁₂), 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 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 toward calculating moderately high-temperature spectra of PAHs and other similarly rigid molecules using Monte Carlo sampling.

中文翻译：

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多环芳烃 (PAHs) 的非谐振动光谱

虽然存在强大的技术来准确解释振动分子光谱中的非谐性，但它们的计算成本非常高，并且不能常规用于大型物种和/或非零振动温度。受空间中多环芳烃 (PAH) 发射研究的启发，我们开发了一种新代码，它考虑了所有模式，可以描述所有红外跃迁，包括由于共振而变得活跃的波段以及泛音、组合和差异波段. 在本文中，我们描述了所实施的方法，并讨论了如何克服主要困难，以使问题易于处理。对小分子进行了高级计算的基准测试。我们对两种原型 PAHs 芘（C₁₆ H ₁₀ ) 和coronene (C ₂₄ H ₁₂ )，旨在优化可调参数以实现此类分子可接受的精度和计算成本。然后，我们报告了在 0 K 下芘和 Coronene 获得的结果，将计算的光谱与可用的实验数据进行了比较。与谐波密度泛函理论计算相比，发现理论带位置显着改善。带强度与实验合理一致，主要限制是四次力场基础计算的准确性。这是使用蒙特卡罗采样计算 PAH 和其他类似刚性分子的中高温光谱的第一步。