Structures, energetics, vibrational spectra of NH4+(H2O)n=4,6 clusters:: Ab initio calculations and first principles molecular dynamics simulations

Important structural isomers of NH4+(H2O)(n=4,6) have been studied by using density functional theory, Moller-Plesset second order perturbation theory, and coupled-cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. The zero-point energy (ZPE) correction to the complete basis set limit of the CCSD(T) binding energies and free energies is necessary to identify the low energy structures for NH4+(H2O)(n=4,6) because otherwise wrong structures could be assigned for the most probable structures. For NH4+(H2O)(6), the cage-type structure, which is more stable than the previously reported open structure before the ZPE correction, turns out to be less stable after the ZPE correction. In first principles Car-Parrinello molecular dynamics simulations around 100 K, the combined power spectrum of three lowest energy isomers of NH4+(H2O)(4) and two lowest energy isomers of NH4+(H2O)(6) explains each experimental IR spectrum. (C) 2008 American Institute of Physics.