Negative velocity correlation in hard sphere fluid

Abstract

Two methods are used to calculate the velocity correlation function K(t) in a hard spheres fluid. The first is a conventional impact theory but with a non-Poissonian distribution of collision times found in simulation experiments. Although not large the negative loop in K(t) appears when the tagged molecule is surrounded by heavier neighbors and back reflection dominates. At liquid densities the heavy particles screen each other from grazing collisions and as a result there is a strong back reflection of the tagged molecule. An alternative mechanism which is appropriate for the particles of equal mass neglects the non-Poissonian corrections, but regards the velocity turns as a highly correlated Markovian process. This results in long-lived circulations inside the liquid cell as in an imperfect ring billiard. The extent of the negative velocity correlation and location of its minimum are described well in this model, which gives the most reliable explanation for quasiperiodic velocity relaxation exhibited in different spectroscopic manifestations.