## Abstract

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.

Original language | English |
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Pages (from-to) | 3986-3997 |

Number of pages | 12 |

Journal | Journal of Chemical Theory and Computation |

Volume | 14 |

Issue number | 8 |

DOIs | |

State | Published - Aug 14 2018 |