Manifestation of the collective drift of molecules in argon according to   their mean square displacements
  The mean square displacement (MSD) of an argon molecule as a function of time is studied. Its deviations from the standard asymptotic law for intermediate times are analyzed in details. It is shown that these deviations are mainly connected with the square-root contribution to the MSD which is proportional the ratio of the collective part to the full self-diffusion coefficient. It is established that the relative value of the collective contribution to the self-diffusion coefficient of argon changes from 0.23 near the triple point up to 0.4 at approaching the critical point. A new method for the determination of the Maxwell relaxation time is proposed. Its temperature dependence on the coexistence curve and one of isochors is investigated. 
Nikolay P. Malomuzh, Konstantin S. Shakun
cond-mat.soft