We present fully coupled, full-dimensional quantum calculations of the inter- and intra-molecular vibrational states of HCl trimer, a paradigmatic hydrogen-bonded molecular trimer. They are performed utilizing the recently developed methodology for the rigorous 12D quantum treatment of the vibrations of the noncovalently bound trimers of flexible diatomic molecules [Felker and Bačić, J. Chem. Phys. 158, 234109 (2023)], which was previously applied to the HF trimer by us. In this work, the many-body 12D potential energy surface (PES) of (HCl)3 [Mancini and Bowman, J. Phys. Chem. A 118, 7367 (2014)] is employed. The calculations extend to the intramolecular HCl-stretch excited vibrational states of the trimer with one- and two-quanta, together with the low-energy intermolecular vibrational states in the two excited v = 1 intramolecular vibrational manifolds. They reveal significant coupling between the intra- and inter-molecular vibrational modes. The 12D calculations also show that the frequencies of the v = 1 HCl stretching states of the HCl trimer are significantly redshifted relative to those of the isolated HCl monomer. Detailed comparison is made between the results of the 12D calculations on the two-body PES, obtained by removing the three-body term from the original 2 + 3-body PES, and those computed on the 2 + 3-body PES. It demonstrates that the three-body interactions have a strong effect on the trimer binding energy as well as on its intra- and inter-molecular vibrational energy levels. Comparison with the available spectroscopic data shows that good agreement with the experiment is achieved only if the three-body interactions are included. Some low-energy vibrational states localized in a secondary minimum of the PES are characterized as well.

中文翻译：

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HCl 三聚体：利用收缩的分子内和分子间碱基进行 12D 全耦合量子计算，得到 HCl 拉伸激发的分子内和分子间振动态

我们提出了 HCl 三聚体（一种典型的氢键分子三聚体）的分子间和分子内振动态的完全耦合、全维量子计算。它们是利用最近开发的方法对柔性双原子分子的非共价键三聚体的振动进行严格的 12D 量子处理 [Felker 和 Bač​​ić, J. Chem.物理。 158, 234109 (2023)]，我们之前已将其应用于 HF 三聚体。在这项工作中，(HCl)3 的多体 12D 势能表面 (PES) [Mancini 和 Bowman, J. Phys.化学。采用 A 118, 7367 (2014)]。计算扩展到具有一量子和二量子的三聚体的分子内 HCl 拉伸激发振动态，以及两个激发 v = 1 分子内振动流形中的低能分子间振动态。它们揭示了分子内和分子间振动模式之间的显着耦合。 12D 计算还表明，HCl 三聚体的 v = 1 HCl 拉伸态的频率相对于分离的 HCl 单体的频率显着红移。将原始 2 + 3 体 PES 中去除三体项后的二体 PES 的 12 维计算结果与 2 + 3 体 PES 的计算结果进行了详细比较。它表明三体相互作用对三聚体结合能及其分子内和分子间振动能级有很强的影响。与现有光谱数据的比较表明，只有包含三体相互作用，才能与实验达到良好的一致性。一些位于 PES 次要极小值的低能振动状态也被表征。