Tests of the Stokes-Einstein Relation through the Shear Viscosity Activation Energy of Water

A method for calculating the activation energy for the shear viscosity of a liquid from simulations at a single temperature is demonstrated. Importantly, the approach provides a route to the rigorous decomposition of the activation energy into contributions due to different motions and interactions, e.g., kinetic, Coulombic, and Lennard-Jones energies, that are otherwise not accessible. The method is illustrated by application to the case of liquid water under ambient conditions. The shear viscosity activation energy and its components are examined and compared to the analogous results for the time scales of diffusion and reorientation that have been previously calculated, providing a test of the Stokes-Einstein relation for water.