Longitudinal and transverse collective dynamics in water by simulation using the BK3 model

Using a recent polarizable model for water (the BK3 model), we explore the collective dynamic modes in liquid water by molecular dynamics (MD) simulation. The dynamic structure factor and the longitudinal and transverse current correlation spectral densities are computed over the whole frequency range below intramolecular excitations. MD results of atom-atom partial correlation functions are fitted using the Generalized Collective Modes (GCMs) model, involving relaxing modes occurring in the longitudinal component and propagating modes occurring in both components. Three systems are studied as follows: (1) BK3 ambient water, (2) SPC/E ambient water, and (3) BK3 ambient heavy water. Comparison between the results of these systems reveals the influence of the polarizability, or the influence of the molecular mass, on the collective dynamics. Moreover, the GCM fitting allows a quantitative description of the excitation modes in terms of the frequencies, damping coefficients and possible coupling between longitudinal and transverse modes. The differences between the three situations are also clearly evidenced within this formalism.