Hydrated proton clusters: Ab initio molecular dynamics simulation and simulated annealing

An ab initio molecular dynamics simulation technique is developed employing the Born–Oppenheimer (BO) approach in the framework of a Gaussian implementation of Kohn–Sham density functional theory (DFT). Simulation results for H5O2+ at 200 K are reported. The density profiles, autocorrelation functions and power spectra are presented. The anharmonic frequencies at 200 K are found to be close to the harmonic frequencies calculated directly from quantum methods at 0 K. Structures of large hydrated proton clusters are optimized. Simulated annealing techniques were employed to search for low energy structures and found to be very useful for clusters with 7–8 water molecules. A few very different structures with ground state energy 1–2 kcal/mol apart are shown. H3O+ is found to be the central unit of a few structures optimized. The ionic hydrogen bond was responsible for the stability of the H9O4+ unit in the large hydrated proton clusters. We also find structures with nascent H5O2+ units at the center whose en...

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