Ab initio prediction of the vibration-rotation-tunneling spectrum of HCl-(H2O)(2)

Quantum calculations of the vibration-rotation-tunneling (VRT) levels of th e trimer HCl-(H2O)(2) are presented. Two internal degrees of freedom are co nsidered-the rotation angles of the two nonhydrogen-bonded (flipping) hydro gens in the complex-together with the overall rotation of the trimer in spa ce. The kinetic energy expression of van der Avoird [J. Chem. Phys. 105, 80 34 (1996)] is used in a slightly modified form. The experimental microwave geometry of Kisiel [J. Chem. Phys. 112, 5767 (2000)] served as input in the generation of a planar reference structure. The two-dimensional potential energy surface is generated ab initio by the iterative coupled-cluster meth od based on singly and doubly excited states with triply excited states inc luded noniteratively [CCSD(T)]. Frequencies of vibrations and tunnel splitt ings are predicted for two isotopomers. The effect of the nonadditive three -body forces is considered and found to be important. (C) 2001 American Ins titute of Physics.