HCl trimer: HCl-stretch excited intramolecular and intermolecular vibrational states from 12D fully coupled quantum calculations employing contracted intra- and intermolecular bases

We present fully coupled, full-dimensional quantum calculations of the inter- and intramolecular vibrational states of HCl trimer, a paradigmatic hydrogen-bonded molecular trimer. They are performed utilizing the recently developed methodology for rigorous 12D quantum treatment of the vibrations of the noncovalently bound trimers of flexible diatomic molecules [P. M. Felker and Z. Bačić, J. Chem. Phys. 158, 234109 (2023)], that was previously applied by us to HF trimer. In this work, the many-body 12D potential energy surface (PES) of (HCl)3 [J. S. Mancini and J. M. 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 intermolecular vibrational modes. The 12D calculations also show that the frequencies of the v = 1 HCl stretching states of 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 2-body PES, obtained by removing the 3-body term from the original 2+3-body PES, and those computed on the 2+3-body PES. It demonstrates that the 3-body interactions have a strong effect on the trimer binding energy as well as on its intra- and intermolecular vibrational energy levels. Comparison with the available spectroscopic data shows that good agreement with experiment is achieved only if the 3-body interactions are included. Some low-energy vibrational states localized in a secondary minimum of the PES are characterized as well.