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/6-311ϩϩG** and MP2/6-311ϩϩG** 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)/6-311ϩϩG** 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.

In this paper we report the results obtained by an implementation and application of the simulated annealing optimization procedure to the exploration of the conformational space of small neutral and charged lithium clusters (Li(n)(q), n = 5, 6, 7; q = 0, +/-1) and of the bimetallic lithium/sodium clusters (Li5Na) in their lowest spin states. Our methodology eliminates the structure guessing procedure in the process of generating cluster configurations. We evaluate the quantum energy, typically with the Hartree-Fock Hamiltonian, of randomly generated points in the conformational space and use a modified Metropolis test in the annealing algorithm to generate candidate structures for atomic clusters. The structures are further optimized by analytical methods (gradient following) at the Møller-Plesset second order perturbation theory level (MP2), in conjunction with basis sets including polarization functions with and without diffuse functions. High accuracy ab initio energies at the coupled clusters level, with single, double, and triple substitutions from the Hartree-Fock determinant (CCSD(T)), on the MP2 geometries were calculated and used to establish the relative stability of the isomers within each potential energy surface. Various cluster properties were computed and compared to existing values in order to validate our methods. Our results show excellent agreement with previous experimental and theoretical reports. Even at these small sizes, evidence for 10 new structures never reported before for the lithium clusters and four new structures for the bimetallic clusters is presented.

Clusters -[calculations for with n = 5-7 and q = 0, ±1, and for Li5Na]. -(PEREZ, J. F.; FLOREZ, E.; HADAD, C. Z.; FUENTEALBA, P.; RESTREPO*, A.; J. Phys. Chem. A 112 (2008) 25, 5749-5755; Inst. Quim., Univ. Antioquia, 1226 Medellin, Colombia; Eng.) -Schramke 39-001 Li q n