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 studied by molecular dynamics simulations. The electronic structure of the system is described by an extended multistate valence-bond Hamiltonian that allows for the breaking and formation of O-H+ bonds. The proton 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+. The proton-transfer mechanism can be described mostly as the translocation of a transient H5O2+ structure across the water hydrogen-bond network. The computed lifetime of a particular H5O2+ is close to 2 ps, a value compatible with experimental estimates.