If there were not a barrier, the jump rate would occur at a frequency of approximately kT/h, where k is Boltzmann’s constant, T is the absolute temperature, and h is Planck’s constant.4 Because of the barrier the jump rate is reduced by a Boltzmann factor. In the absence of any stress (pressure drop), jumps occur equally in both directions over the barrier. However, the presence of a stress makes jumps easier in one direction than another. Calculating the net jump rate in one direction gives the net flow velocity. Using the definition of the viscous force in equation (5.1), this gives the theoretical prediction for the viscosity coefficient