The Gaussian-3 (G3) model chemistry method has been used to calculate the relative deltaG(o) values for all possible conformers of neutral clusters of water, (H2O)n, where n = 3-5. A complete 12-fold conformational search around each hydrogen bond produced 144, 1728, and 20,736 initial starting structures of the water trimer, tetramer, and pentamer. These structures were optimized with PM3, followed by HF/6-31G* optimization, and then with the G3 model chemistry. Only two trimers are present on the G3 potential energy hypersurface. We identified 5 tetramers and 10 pentamers on the potential energy and free-energy hypersurfaces at 298 K. None of these 17 structures were linear; all linear starting models folded into cyclic or three-dimensional structures. The cyclic pentamer is the most stable isomer at 298 K. On the basis of this and previous studies, we expect the cyclic tetramers and pentamers to be the most significant cyclic water clusters in the atmosphere.

ABSTRACT:The Gaussian-3 method developed by Pople and coworkers has been used to calculate the free energy of neutral octamer clusters of water, (H 2 O) 8 . The most energetically stable structures are in excellent agreement with those determined from experiment and those predicted from previous high-level calculations. Cubic structures are favored over noncubic structures over all temperature ranges studied. The D 2d cubic structure is the lowest free energy structure and dominates the potential energy and free energy hypersurfaces from 0 K to 298 K.