To investigate the structures of I-(H2O)(n = 1-6), extensive ab initio calculations have been carried out. Owing to very flexible potential surfaces of the system (in particular for n = 5 and 6), the lowest energy structures are characterized from various possible low-lying energy conformers. In contrast to some previously reported structures, we find a new lowest energy structure (followed by a few low-lying energy conformers) for n = 5 and four nearly isoenergetic conformers for n=6. These conformers have surface and near-surface structures with the coordination number of 4. The present results provide the information of possible structures in recent profuse experiments of infrared spectra of I-(H2O)(n = 1-6) and charge transfer from the excited iodide ion to water molecules. Our predicted ionization potentials and OH stretching frequencies are in good agreement with the experimental data available, while only the cases of the OH frequencies for n = 4 and the ionization potential for n = 5 need consideration of conformational change by the temperature effect. (C) 2001 American Institute of Physics.