  A practical correction formula relating the self-diffusion coefficient of
dense liquids from molecular dynamics simulations with periodic boundary
conditions to the self-diffusion coefficient in the thermodynamic limit is
discussed. This formula applies to pure dense fluids and has a very simple form
$D=D_0(1-\gamma N^{-1/3})$, where $D_0$ is the self-diffusion coefficient in
the thermodynamic limit and $N$ is the number of particles in the simulation.
The numerical factor $\gamma$ depends on the geometry of the simulation cell.
Remarkably, $\gamma\simeq 1.0$ for the most popular cubic geometry. The success
of this formula is supported by results from MD simulations, including very
recent simulations with a ``magic'' simulation geometry.
