An ab initio study of the (H2O)20H+ and (H2O)21H+ water clusters

The study of the minimum Born-Oppenheimer structures of the protonated water clusters, (H2O)(n)H+, is performed for n=20 and 21. The structures belonging to four basic morphologies are optimized at the Hartree-Fock, second-order many-body perturbation theory and coupled cluster level, with the 6-31G, 6-31G*, and 6-311G** basis sets, using the parallel ACES III program. The lowest energy structure for each n is found to be the cagelike form filled with H2O, with the proton located on the surface. The cage is the distorted dodecahedron for the 21-mer case, and partially rearranged dodecahedral structure for the 20-mer. The results confirm that the lowest energy structure of the magic number n=21 clusters corresponds to a more stable form than that of the 20-mer clusters. (C) 2009 American Institute of Physics. [doi:10.1063/1.3231684]