On the memory function for the dynamic structure factor of interacting brownian particles

It is shown that the memory function of concentration fluctuations M̃(k, t), derived from the generalized Smoluchowski equation, is not a one-particle irreducible quantity. A new function M(k, t), which is one-particle irreducible, is derived and the relation between both functions is established. The irreducible memory function M(k, t) can be interpreted as a dynamic viscosity. In contrast to earlier statements there is no discrepancy in the results, which are obtained from phase space approach (Fokker-Planck equation) and those obtained from the Smoluchowski equation.