Fractional Stokes-Einstein relation in TIP5P water at high temperatures

Fractional Stokes-Einstein relation described by D~(τ/T ight)^ξ is observed in supercooled water, where D is the diffusion constant, τ the structural relaxation time, T the temperature, and the exponent ξ≠-1. In this work, the Stokes-Einstein relation in TIP5P water is examined at high temperatures within 400 K-800 K. Our results indicate that the fractional Stokes-Einstein relation is explicitly existent in TIP5P water at high temperatures, demonstrated by the two usually adopted variants of the Stokes-Einstein relation, D~τ^-1and D~T/τ, as well as by D~T/η, where η is the shear viscosity. Both D~τ^-1 and D~T/τ are crossed at temperature T_x=510 K. The D~τ^-1 is in a fractional form as D~τ^ξ with ξ=-2.09 for T ≤ T_x and otherwise ξ=-1.25. The D~T/τ is valid with ξ=-1.01 for T ≤ T_x but in a fractional form for T> T_x. The Stokes-Einstein relation D~T/η is satisfied below T_x=620 K but in a fractional form above T_x. We propose that the breakdown of D~T/η may result from the system entering into the super critical region, the fractional forms of D~τ^-1 and D~T/τ are due to the disruption of the hydration shell and the local tetrahedral structure as well as the increase of the shear viscosity.