  Recently, an analytical expression for the system size dependence and
direction-dependence of self-diffusion coefficients for neat liquids due to
hydrodynamic interactions has been derived for molecular dynamics (MD)
simulations using orthorhombic unit cells. Based on this description, we show
that for systems with a "magic" box length ratio of
$L_z/L_x\!=\!L_z/L_y\!=\!2.7933596497$ the computed self-diffusion coefficients
$D_x$ and $D_y$ in $x$- and $y$-direction become system-size independent and
represent the true self-diffusion coefficient $D_0\!=\!(D_x+D_y)/2$. Moreover,
by using this particular box geometry, the viscosity can be determined with a
reasonable degree of accuracy from the difference of components of the
diffusion coefficients in $x$-,$y$- and $z$-direction using the simple
expression $\eta\!=\!k_\mathrm{B}T\cdot 8.1711245653/[3\pi
L_z(D_{x}+D_{y}-2D_z)]$, where $k_\mathrm{B}$ denotes Boltzmann's constant, and
$T$ represents the temperature. MD simulations of TIP4P/2005 water for various
system-sizes using both orthorhombic and cubic box geometries are used to test
the approach.
