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

Monte Carlo sampling calculations were performed to determine the anharmonic sum of states, N _anh ( E), for I ^- (H ₂ O), (H ₂ O) ₂ , and I ^- (H ₂ O) ₂ versus internal energy up to their dissociation energies. The anharmonic density of states, ρ _anh ( E), is found from the energy derivative of N _anh ( E). Analytic potential energy functions are used for the calculations, consisting of TIP4P for H ₂ O···H ₂ O interactions and an accurate two-body potential for the I ^- ···H ₂ O 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 ^- (H ₂ O) 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, ρ _anh ( E)/ρ _h ( E), is ∼2, ∼ 3, and ∼260 for I ^- (H ₂ O), (H ₂ O) ₂ , and I ^- (H ₂ O) ₂ , respectively. The large ratio for I ^- (H ₂ O) ₂ results from the I ^- (H ₂ O) ₂ → I ^- (H ₂ O) + H ₂ O dissociation energy being more than 2 times larger than the (H ₂ O) ₂ → 2H ₂ O dissociation energy, giving rise to highly mobile H ₂ O molecules near the I ^- (H ₂ O) ₂ dissociation threshold. This work illustrates the importance of treating anharmonicity correctly in unimolecular rate constant calculations.