Quantum statistical mechanical simulation of the ion-water cluster I-(H2O)(n): The importance of nuclear quantum effects and anharmonicity

Monte Carlo simulations of quantum statistical mechanical properties using the Feynman path integral method were carried out at temperatures of 70, 100, 200, and 300 K to study the structure and energetics of the ion-water cluster I- (H2O)(n), with n = 1-6. Simulation results at low temperatures (i.e., 70 K) can be directly compared with data from photoelectron spectroscopy (PES). The temperature dependence of binding enthalpies and stabilization energies indicate that at low temperatures the classical description of nuclear motion is qualitatively incorrect. The binding enthalpies are also computed within the harmonic approximation using the energy and frequencies at the minimum energy geometry, a method widely used in conjunction with electronic structure calculations to extend zero temperature information to finite temperatures. It is found that the harmonic approximation works well for the cluster with a single water molecule, but not for clusters with more than one water molecule, where the temperature dependence of binding enthalpies is in qualitative disagreement with the exact enthalpy obtained from simulation. (C) 1996 American Institute of Physics.