Comment on "reversing the perturbation in nonequilibrium molecular dynamics: an easy way to calculate the shear viscosity of fluids".

Müller-Plathe [Phys. Rev. E 59, 4894 (1999)] gives a velocity exchange method that creates a shear momentum flux. Measurement of the mean velocity gradient allows the determination of the shear viscosity. Low gradients are achieved when the time interval between exchanges is large (low frequency). We show that low frequency does not produce a steady shear flow while a weaker, but continuous exchange, does have a steady flow.