Mass dependence of isotope self-diffusion by molecular dynamics.

Abstract

The mass and system size dependence of the self-diffusion coefficient of a single solute particle in a Weeks-Chandler-Anderson and Lennard-Jones liquid have been determined for a wide range of solute particle mass ${\mathit{m}}_{\mathit{B}}$ and the number of particles of the host fluid N-1. The self-diffusion coefficient has been calculated using the mean squared displacements, the velocity autocorrelation function of the particles, and the Gaussian memory function method. From the computer simulation results, we conclude that only for Brownian particles with a mass less than a critical value (ca. ${\mathit{m}}_{\mathit{B}}$/m25, where m is the mass of the solvent molecule) and very small systems (ca. N500) is it possible to detect a mass and system-size dependence of the self-diffusion coefficient. For more massive Brownian particles and larger systems, the self-diffusion coefficient of the Brownian particle reaches a thermodynamic limit, neither depending on its mass nor on the number of particles in the periodic solvent system. The results obtained suggest that the self-diffusion coefficient for sufficiently large isotope masses should only depend on the temperature and number density of the fluid particles.