Abstract
In the present study, molecular dynamics (MD) simulations on monatomic Lennard-Jones and water bulk liquids in the periodic boundary system were performed in order to elucidate the effect of the system size and shape on self-diffusion coefficient. So far, the system size effect in cubic computational domains has been intensively investigated and these studies showed that the diffusion coefficient becomes smaller as the system size decreases. We examined the system size effect not only in the cubic cell systems, but also in the rectangular cell systems which were created by changing one side length of the cubic cell with the system density kept. As a result, the diffusion coefficient in the direction perpendicular to the longer side of the rectangular cell significantly increases more or less linearly as the side length increases for both liquids. On the other hand, the diffusion coefficient in the direction along the longer side is almost constant or slightly decreases. Consequently, anisotropy of diffusion coefficient appears in the rectangular cell with a periodic boundary condition even in the case of bulk liquids.
