Roles of translational and reorientational modes in translational diffusion of high-pressure water: comparison with...

The dynamics of two soft-core fluids that show the increase in diffusivity with isothermal compression is studied with the mode-coupling theory (MCT). The anomalous density dependence of the diffusivity of these fluids is reproduced by the theory, and it is ascribed to the decrease in the first peak of the structure factor. The mechanism is quite different from that of high-pressure water revealed by MCT on molecular liquids described by the interaction-site model [T. Yamaguchi, S.-H. Chong, and F. Hirata, J. Chem. Phys., 119, 1021 (2003)]. The structures used in that study, calculated by the reference interaction-site model integral equation theory, showed the increase in the height of the first peak of the structure factor between oxygen atoms, whereas the structure obtained by molecular dynamics (MD) simulations shows the decrease in the peak height. In this work, calculations with MCT are performed on the simple fluids whose structure factor is the same as that between oxygen atoms of water from MD simulation, in order to clarify the role of translational structure on the increase in diffusivity with compression. The conclusion is that both the translational and reorientational modes contribute to the increase in diffusivity, and the effect of the latter is indispensable for the anomaly alone at least above freezing temperature.