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 – (H 2 O) and F – (H 2 O) 2 using full-dimensional potential energy and dipole moment surfaces. The full-dimensional F – (H 2 O) 2 potential energy surface is composed of previously published one- and two-body water potentials, a two-body F – H 2 O potential, and a new three-body F – (H 2 O) 2 potential. The three-body water–fluoride potential is a fit to 16 111 second-order Møller–Plesset perturbation theory (MP2/aug-cc-pVTZ) energies using permutationally invariant polynomials. The IR spectrum of F – (H 2 O) 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.