A study was made of the system-size dependence of translational diffusion coefficients and viscosities in molecular dynamics simulations under periodic boundary conditions. Simulations of water under ambient conditions and a Lennard-Jones fluid showed that the diffusion coefficients increased strongly as the system size increased. A simple analytic correction for the system-size effects was tested that was based upon hydrodynamic arguments. This correction scaled as N-1/3, where N was the number of particles. For a cubic simulation box of length L, the diffusion coefficient corrected for system-size effects was D0 = DPBC + 2.837297kT/(6πηL), where DPBC was the diffusion coefficient calculated in the simulation and η the shear viscosity of the solvent. For water, Lennard-Jones fluids, and hard-sphere fluids, this correction quantitatively accounted for the system-size dependence of the calculated self-diffusion coefficients. In contrast to diffusion coefficients, the shear viscosities of water and the Lennard-Jones fluid exhibited no significant system-size dependences.

System-Size Dependence of Diffusion Coefficients and Viscosities from Molecular Dynamics Simulations with Periodic Boundary Conditions. Yeh, I.C., Hummer, G.: Journal of Physical Chemistry B, 2004, 108[40], 15873-9