Vibration-rotation-tunneling dynamics of HCl and HF dimers from full-dimensional quantum calculations

We report accurate quantum mechanical calculations of the rovibrational levels of HCl and HF dimers, for total angular momentum J = 0, 1, and for HX (X = Cl,F) monomers in the ground and excited vibrational states. All six inter- and intra-molecular degrees of freedom of the complexes are treated as fully coupled without any dynamical approximation. The six-dimensional (6D) basis used in the calculations takes advantage of the total parity and exchange symmetry present in these systems, leading to block diagonalization of the Hamiltonian into four distinct blocks. The symmetry blocks are diagonalized separately, reducing greatly the computational effort.; Our calculations provide a comprehensive description of the bound state properties of (HCl){dollar}sb2{dollar} and (HF){dollar}sb2{dollar}, including their dissociation energies, intramolecular vibrational frequencies for various HX-stretch excited manifolds, and mode specificity of tunneling splittings. Direct comparison with high-resolution experiments allows unambiguous assessment of the quality of the 6D potential energy surfaces (PESs) employed, as well as interpretation and assignment of the experimental spectra. Our calculations for (HCl){dollar}sb2{dollar} have revealed that the two most widely used PESs, which predict reasonably well the spectroscopy of the dimer for ground-state HCl monomers, yield zero tunneling splitting when one of the monomers is vibrationally excited, in sharp contrast with experimental results. This heretofore unsuspected deficiency of the PESs was corrected in part by the addition of an electrostatic interaction potential, which depends on the vibrational coordinates of the two monomers. A couple of new ab initio-based 6D PESs for (HF){dollar}sb2{dollar} have been tested by means of rovibrational J = 0, 1 calculation. Comparison of our theoretical results with experiment shows that the new PESs represent a significant improvement over the best previously available potentials.