Flexible molecules and non‐idle environments can present severe problems in the prediction of vibrational spectra. This work focuses on infrared and vibrational circular dichroism for which the latter is extremely sensitive to such complicated situations. Here two possible ways to perform spectroscopy calculations for such situations are investigated. The first approach is based on static quantum chemical calculations employing cluster‐weighting. The second approach is rooted in ab initio molecular dynamics simulations using the time correlations approach. For the present example ((R)‐butan‐2‐ol), excellent spectra from simulations are obtained for gas and bulk, when the former is averaged over trajectories with all possible starting conformers and these are scaled to match the experimental spectrum. The cluster‐weighted approach is inferior to the simulations but still reaches very good results at much less computational cost. A simplified, but computationally less expensive simulation approach is considered by approximating the electric and magnetic moments, which are input quantities in the correlation function, by classical equations and different populations analysis for the required partial charges. The results are inferior to the full simulations but still give satisfying results at the advantage of being much faster to calculate.

中文翻译：

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预测柔性分子和非空闲环境下的分子的振动光谱

柔性分子和非空闲环境在振动光谱的预测中可能会出现严重问题。这项工作的重点是红外和振动圆二色性，而后者对于这种复杂情况极为敏感。在此，研究了针对这种情况执行光谱计算的两种可能方法。第一种方法基于采用簇加权的静态量子化学计算。第二种方法植根于使用时间相关性方法进行的从头算分子动力学模拟。对于本示例（（R）-丁-2-醇），如果将前者在所有可能的起始构象异构体的轨迹上进行平均，并根据实验光谱进行缩放，则可以从模拟中获得出色的气体和本体光谱。聚类加权方法不如模拟方法，但仍以较少的计算成本获得了很好的结果。通过用经典方程式和对所需部分电荷的不同总体分析来近似作为相关函数中输入量的电磁矩和磁矩，可以考虑一种简化但计算成本较低的仿真方法。结果不如完整的模拟，但仍具有令人满意的结果，其优点是计算速度更快。