Models for the description of the H3O+ and OH- ions in water

Structures of the H3O+-OH- ion pair surrounded by up to three water molecules have been studied. Since the ion-pair structure is always above the corresponding neutral water structure, a constrained geometry optimization is needed. The energy difference between the ion-pair structure and the neutral water structure is studied as a function of the number of surrounding water molecules. The effect of the surrounding water solvent is also studied by placing the model system in a spherical cavity in a dielectric medium. The main results are that the energy difference stabilizes at 10-20 kcal/mol for the larger clusters and that an effect indicating a mechanism for charge separation can be noticed on the geometries of these clusters. Results obtained using gradient-corrected density-functional theory are compared to a configuration interaction treatment using a scaling procedure of the correlation energy. (C) 1996 by John Wiley & Sons, Inc.