HCl-HO dimer: an accurate full-dimensional potential energy surface and fully coupled quantum calculations of intra- and intermolecular vibrational states and frequency shifts.

The interaction between HCl and HO is of considerable theoretical and experimental interest due to its important role in atmospheric chemistry and understanding the onset of the dissociation of HCl in water. In this work, the HCl-HO complex is quantitatively characterized in two ways. First, we report a new full-dimensional potential energy surface (PES) for the HCl + HO system. The nine-dimensional (9D) PES is based on circa 43 000 ab initio points calculated at the level of CCSD(T)-F12a/AVTZ with the basis set superposition error correction using the permutation invariant polynomial-neural network method, which can accurately and efficiently reproduce the geometries, energies, frequencies of the complex of HCl with HO, as well as the relevant minimum energy path. Next, we present the results of the first fully coupled 9D quantum calculations of the intra- and intermolecular vibrational states of the HCl-HO dimer, performed on the new PES. They employ the highly efficient bound-state methodology previously used to compute accurately the rovibrational level structure of the HO/DO-CO and HDO-CO complexes [P. M. Felker and Z. Bačić, J. Chem. Phys., 2020, 153, 074107; J. Phys. Chem. A, 2021, 125, 980]. The 9D calculations characterize the vibrationally averaged nonplanar ground-state geometry of the HCl-HO complex, the intramolecular vibrational fundamentals of both HO and HCl moieties, and their frequency shifts, as well as the low-energy intermolecular vibrational states in each of the intramolecular vibrational manifolds and the effects of the coupling between the two sets of modes. The calculated properties of the HCl-HO dimer are in excellent agreement with the available spectroscopic data. The 9D computed dimer binding energy D of 1334.63 cm agrees extremely well with the experimental D equal to 1334 ± 10 cm [B. E. Casterline and A. K. Mollner and L. C. Ch'ng and H. Reisler, J. Chem. Phys., 2010, 114, 9774]. Moreover, the ground-state expectation value of the out-of-plane bend angle of HO, 33.80°, and the computed HCl stretch frequency shift, -157.9 cm, both from the 9D calculations, are in very good accord with the corresponding experimental values.