Ionic hydrogen-bond networks and ion solvation. 1. An efficient Monte Carlo/quantum mechanical method for structural search and energy computations: ammonium/water.

Abstract

Hydrogen-bond networks about solvated ions can form many alternative structures, requiring extensive conformational searches with accurate but affordable energy computation. For this purpose we are combining Monte Carlo searches with a computationally efficient density-functional-based tight-binding (DFTB+) method. The approach is tested for the stepwise hydration energies of the ammonium ion in NH(4)(+)(H(2)O)(n) clusters (n = 1-8), for which experimental data are available. For each cluster size n, we perform Monte Carlo searches, where for each conformation we calculate the minimized energy using the DFTB+ method (and, for comparison, using MM3 or OPLS-AA force field). The Monte Carlo/DFTB+ search identifies the lowest energy structure that can be reoptimized with other quantum mechanical methods (here with HF, B3LYP, B3PW91, MP2, DFT, and CBS-Q, CBS-QB3, and CBS-APNO methods). Calculated geometries and charge densities for the clusters are also presented. The results show that the binding energies calculated by the DFTB+ method reproduce the values measured experimentally and predicted by highly correlated, but significantly more computationally intensive, ab initio quantum chemical methods. The encouraging results suggest that the Monte Carlo/DFTB+ approach is a computationally efficient quantum chemical method for relatively large solvated systems, as demonstrated here for cluster ions.

DOI: 10.1021/jp808486k

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Cite this paper

@article{Zhao2009IonicHN, title={Ionic hydrogen-bond networks and ion solvation. 1. An efficient Monte Carlo/quantum mechanical method for structural search and energy computations: ammonium/water.}, author={Yi-Lei Zhao and Michael Meot-Ner Mautner and Carlos A Gonzalez}, journal={The journal of physical chemistry. A}, year={2009}, volume={113 12}, pages={2967-74} }