Theoretical predictions of vibration–rotation–tunneling dynamics of the weakly bound trimer (H2O)2HCl

In this Letter we report a theoretical study of the vibration–rotation–tunneling (VRT) states of the (H2O)2HCl trimer. Five degrees of freedom are considered: two angles corresponding to the torsional (flipping) motions of the free, non-hydrogen-bonded, hydrogen atoms in the complex, and three angles describing the overall rotation of the trimer in the space. A two-dimensional potential energy surface is generated ab initio by symmetry-adapted perturbation theory (SAPT). Tunneling splittings, frequencies of the intermolecular vibrations, and vibrational line strengths of spectroscopic transitions are predicted.