title: 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.
authors.affiliation: Chemical and Biochemical Reference Data Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA. yi-lei.zhao@nist.gov
authors.forename: Yi-Lei
authors.initials: YL
authors.lastname: Zhao
authors.forename: Michael
authors.initials: M
authors.lastname: Meot-Ner Mautner
authors.forename: Carlos
authors.initials: C
authors.lastname: Gonzalez
mesh_terms: D000641:Ammonia / Q000737:chemistry*; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry*; D015394:Molecular Structure; D009010:Monte Carlo Method; D011789:Quantum Theory; D012997:Solvents; D014867:Water / Q000737:chemistry*
chemical_list: D007477:Ions; D012997:Solvents; D014867:Water; D000641:Ammonia