Transverse current and generalized shear viscosity in liquid rubidium

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Abstract
The transverse current correlation function is studied in liquid rubidium by computer simulation, and the associated memory function is directly determined from the data, for a number of wave vectors. The usual phenomenological relaxation-time approximations for the memory function are shown to be inadequate, particularly in the wave-vector range where well-defined shear waves are supported by the liquid. A comparison of the results with new data obtained for a Lennard-Jones system is also made, and the dynamical processes contributing to the structure of the memory functions are assessed. The related generalized wave-vector-dependent shear viscosity, η(q), is derived and its importance to the applicability of a microscopic Stokes-Einstein relation finally discussed. It is shown that the details in η(q), which are revealed by the molecular-dynamics data, play a vital role in establishing this relation in liquid rubidium.