Anharmonic Densities of States for Vibrationally Excited I-(H2O), (H2O)2, and I-(H2O)2.

Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, Nanh( E), for I-(H2O), (H2O)2, and I-(H2O)2 versus internal energy up to their dissociation energies. The anharmonic density of states, ρanh( E), is found from the energy derivative of Nanh( E). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H2O···H2O interactions and an accurate two-body potential for the I-···H2O fit to quantum chemical calculations. The extensive Monte Carlo samplings are computationally demanding, and the use of computationally efficient potentials was essential for the calculations. Particular emphasis is directed toward I-(H2O)2, and distributions of its structures versus internal energy are consistent with experimental studies of the temperature-dependent vibrational spectra. At their dissociation thresholds, the anharmonic to harmonic density of states ratio, ρanh( E)/ρh( E), is ∼2, ∼ 3, and ∼260 for I-(H2O), (H2O)2, and I-(H2O)2, respectively. The large ratio for I-(H2O)2 results from the I-(H2O)2 → I-(H2O) + H2O dissociation energy being more than 2 times larger than the (H2O)2 → 2H2O dissociation energy, giving rise to highly mobile H2O molecules near the I-(H2O)2 dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations.