ENERGETICS OF PROTON-TRANSFER IN LIQUID WATER .1. AB-INITIO STUDY FOR ORIGIN OF MANY-BODY INTERACTION AND POTENTIAL-ENERGY SURFACES

The energetics of proton transfer in liquid water is investigated by using ab initio calculation. The molecular electronic interaction of hydrated proton clusters is classified into many-body interaction elements by a new energy decomposition method. It is found that up to three-body molecular interaction is essential to describe the potential energy surface. The three-body effect mainly arises from the (non-classical) charge transfer and strongly depends on their configuration. Higher than three-body effects are small enough to be neglected. To simulate the liquid state reactions, two cluster models including all water molecules up to the second shell in the proton transfer reactions are employed. It is shown that these proton transfer reactions only involve small potential energy barriers of a few kcal/mol or less when structural rearrangement of the solvent is induced along the proton movement.