QUANTUM DYNAMICS OF AN EXCESS PROTON IN WATER USING AN EXTENDED EMPIRICAL VALENCE-BOND HAMILTONIAN

The classical and quantum dynamics of an excess proton in water is stu died by molecular dynamics simulations. The electronic structure of th e system is described by an extended multistate valence-bond Hamiltoni an that allows for the breaking and formation of O-H+ bonds. The proto n quantum character is treated by means of an effective (path-integral ) proton-transfer surface. Whereas classical simulations predict that the hydrated proton appears in a mixture of H5O2+ and H9O4+ structures , inclusion of proton quantization leads to the prevalence of H5O2+. T he proton-transfer mechanism can be described mostly as the translocat ion of a transient H5O2+ structure across the water hydrogen-bond netw ork. The computed lifetime of a particular H5O2+ is close to 2 ps, a v alue compatible with experimental estimates.