Structural studies of the water tetramer

in this article, we report the geometries and properties of the structural isomers obtained from a random walk of the potential energy surface of the water tetramer. Eight structures were obtained after B3LYP/6-311++G** and MP2/6-311++G** optimization of 59 candidate structures generated from a stochastic search of the PM3 conformational space. The random search was carried out using a recently proposed modified Metropolis acceptance test in the simulated annealing procedure. Highly correlated CCSD(T)/6-311++G** energies on the MP2 geometries were calculated and used to determine relative stabilities and to estimate isomer concentrations. We obtained three different geometrical motifs: planar cyclic tetramers, planar cyclic trimers interacting with an extra off-plane water molecule, and a bicycle-like structure. Our CCSD(T) energies, in agreement with previous reports, predict the global minimum to be a planar cyclic tetramer with hydrogen atoms alternating above and under the plane. (c) 2008 Wiley Periodicals, Inc.