Exact six-dimensional quantum calculations of the rovibrational levels of (HCl)₂

Results of comprehensive full-dimensional (6D) quantum calculations of the rovibrational levels of (HCl)₂, for total angular momentum J = 0,1 are presented. The calculations employed two 6D potential energy surfaces (PES) - the ab initio PES of Bunker and co-workers, and the semiempirical PES of Elrod and Saykally. This 6D study provides the first rigorous, approximation-free description of the bound state properties of (HCl)₂, including the dissociation energy, tunneling splittings and their J, K dependence, frequencies of intermolecular vibrations and associated J = 0→1 spacings, and quantum number assignments of the 6D eigenstates. Detailed comparison with 4D bound state calculations (for fixed HCl bond length) was made in order to assess the importance of including the intramolecular vibrations of the two HCl subunits for accurate calculation of various spectroscopic properties of (HCl)₂. Comparison of the 6D results with experimental data, while confirming that the ES1 PES is substantially more accurate than the ab initio PES, shows that there is room for further refinements, preferably using 6D bound state calculations.