Zero temperature quantum properties of small protonated water clusters (H2O)nH+ (n=1–5)

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The study of the energetics and structure of small protonated water clusters (H2O)nH+ (n=1–5) has been carried out employing the OSS3 potential energy surface developed by Ojamae, Singer, and Shavitt [J. Chem. Phys. 109, 5547 (1998)]. By comparing it with accurate ab initio MP2 calculations for (H2O)nH+, this all-atom potential is also shown to reproduce quantitatively the geometry and the relative energetics of small neutral and protonated water clusters containing up to five molecules. To correct the total and binding energy for vibrational motion, the zero point energy of the clusters has been calculated by means of the harmonic approximation and by simulating the exact ground state using the diffusion Monte Carlo method. From these 0 K results, it appears that the anharmonicity accounts for a decrease (increase) of 1.5–5.5 mhartree (1.0–3.5 kcal/mol) in the total (binding) energy of the protonated clusters. Moreover, we found all the cyclic isomers of (H2O)4H+ and (H2O)5H+ to be unstable during the diffusion Monte Carlo simulations, and to convert into treelike or linear isomers. Employing the same interaction potential, we also simulated the ground state of (H2O)n (n=1–5) to compute the proton binding energy to a water cluster. This quantity is decreased by roughly 12 mhartree (7.5 kcal/mol) by including the zero point energy correction to the total energy. The relevance of these findings with respect to the experimental detection and probing of the protonated water clusters is discussed.