SELF-DIFFUSION OF LARGE SOLID CLUSTERS IN A LIQUID BY MOLECULAR-DYNAMICS SIMULATION

Molecular dynamics simulations have been carried out of rigid and dyna mic solid near-spherical atomistically discrete Lennard-Jones (LJ) clu sters in a WCA host liquid. A single duster consisted of 5-256 LJ part icles in systems containing up to 27000 particles in total. The diffus ion coefficients were found to be insensitive to the nature of the deg rees of freedom of the cluster. Rigid clusters, with no internal degre es of freedom, gave essentially the same self-diffusion coefficients a s those composed of thermally interacting LJ atoms. The diffusion coef ficients decrease with cluster size and increase with system size. For clusters in excess of 100 particles, system sizes of at least 10 000 particles are required to attain the thermodynamic limit. In the therm odynamic limit, the Stokes-Einstein relationship is obeyed approximate ly, assuming an increase in the local viscosity of the liquid around t he cluster, as a consequence of an observed enhanced local order in th is region. We have shown that clusters of several hundred atoms exhibi t classical Langevin dynamics, with near exponential long-term decay o f the force and velocity autocorrelation functions. The large clusters exhibit slow reorientational relaxation compared with that of angular frequency.