AN EXTENDED BASIS-SET AB-INITIO STUDY OF LI-6( (H2O)N, N=1)

The structures, binding energies, and enthalpies of small molecular cl usters incorporating a single lithium cation and up through six waters have been determined with extended Gaussian basis sets using Hartree- Fock and post-Hartree-Fock methods. The resulting properties are analy zed with respect to both basis set completeness and degree of correlat ion recovery, including core-core and core-valence effects. Although t he lithium-water interaction is largely electrostatic in nature, small basis sets, lacking in polarization and near-valence diffuse function s, drastically overestimate the strength of the bond (by 20 kcal/mol o r more) and underestimate the Li+...O distance by up to 0.1 angstrom. Their poor performance is attributable to inherent errors in describin g the electric moments and polarizability of water and to large basis set superposition errors. Thus, the accuracy with which the fundamenta l lithium-water interaction could be modeled was primarily dependent o n the quality of the Gaussian basis set and not upon the level of corr elation recovery. Basis set enlargement and correlation effects both t end to reduce the strength of the Li+(H2O) bond, but produce correctio ns of opposite sign for the Li+...O bond length. Although correlation effects play a minor role in describing the lithium-water interaction, as the size of the cluster increases and the number of waters involve d in multiple hydrogen bonds grows, correlation recovery can become si gnificant.