Coupling of Low- and High-Frequency Vibrational Modes: Broadening in the Infrared Spectrum of F-(H2O)2

We present ab initio calculations of the infrared (IR) spectra of the strongly anharmonic clusters F-(H2O) and F-(H2O)(2) using full-dimensional potential energy and.:dipole moment surfaces; The full-dimensional F-(H2O)(2) potential energy surface is composed of previously published one and two body water potentials, a two body F-H2O potential; and a new three body F-(H2O)(2), potential. The three body water-fluoride potential is a fit to 16 111 second-order Moller-Pleiset perturbation theory (MP2/aug-cc-pVTZ) energies using permutationally invariant polynomials. The IR spectrum of F-(H2O)(2) is computed using 6-, 8, 10-, and 13-dimensional VSCF/VCI in order to illustrate the effects of mode coupling and intensity borrowing between the high-frequency intramolecular modes and the low-frequency intermolecular modes. Comparisons with Ar-tagged action spectra are also made.