NON-GAUSSIAN BEHAVIOR AND THE DYNAMICAL COMPLEXITY OF PARTICLE MOTIONIN A DENSE 2-DIMENSIONAL LIQUID

The single particle dynamics of a 2D liquid made up of soft disks inte racting by a repulsive r(-12) potential are studied using molecular dy namics simulations. We find that mean squared particle displacement [D elta r(t)(2)] behaves diffusively, i.e., increases linearly with time, within a time interval t(c) very much shorter than that required for structural relaxation. The non-Gaussian parameter alpha(t)=[Delta r(4) ]/2[Delta r(2)](2)-1, on the other hand, exhibits a significant peak a t times considerably greater than t(c) and a subsequent slow decay. It is argued that the only picture of diffusion consistent with these re sults considers particles moving in a medium characterized by fluctuat ing local mobilities. This picture provides an explicit connection bet ween structural fluctuations (as characterized by the local mobility) and single particle motion. The possibility of obtaining the width and lifetime of the distribution of local relaxation times from incoheren t scattering is examined. (C) 1996 American Institute of Physics.