Molecular dynamics of an excess proton in water using a non-additive valence bond force field

We introduce a valence-bond-based semi-empirical Hamiltonian for describing the dynamics of an excess proton in water. The model includes an arbitrary number N of valence states and the ground electronic state is obtained by diagonalization of the resulting N x N valence bond matrix. The force field thus obtained is non-additive and allows for the delocalization of the excess protonic charge over several surrounding water molecules. This force field is used to perform simulated annealing optimizations and molecular dynamics simulations at finite temperatures in order to investigate the structural and dynamical properties of small H+(H2O)(n) clusters, as well as proton transfer dynamics along a H13O6+ structure in liquid water. (C) 1997 Elsevier Science B.V.