APPLICABILITY OF SEMIEMPIRICAL METHODS TO THE STUDY OF SMALL WATER CLUSTERS - CUBIC STRUCTURES FOR (H2O)(N) (N = 8, 12, 16)

Although up to 15 different crystalline structures for water ice have been observed, the most stable form, ice I-h, has a hexagonal structur e. This structure, in which each oxygen is surrounded by four other ox ygens arranged in a regular tetrahedron, maximizes the number of hydro gen bonds for the system, while accommodating the structure of the wat er monomer with minimal strain. Small clusters of water cannot achieve maximum hydrogen bonding without introducing both non-linear hydrogen bonds and some distortion of the monomer geometry. Thus it is of cons iderable interest that the lowest energy structure for (H2O)(8) predic ted by both ab initio calculations and the TIP4P empirical potential h as cubic-like D-2d symmetry (C.J. Tsai, J.D. Jordon, J. Phys. Chem. 97 (1993) 5208). For the 8-mers and next larger clusters (n = 8, 12), th e energy ordering of the structures predicted using the TIP4P potentia l generally agrees with that predicted by MP2. Because of the potentia l utility of semi-empirical calculations for studying growth processes for water clusters and ice lattices, this study was undertaken to det ermine whether the cubic structures were predicted to be stable using semi-empirical methods. The energies and structures for water clusters predicted using MNDO-PM3 are compared with ab initio, empirical and e xperimental results. (C) 1997 Elsevier Science B.V.