  Cryogenic buffer gas cells have been a workhorse for the cooling of molecules
in the last decades. The straightforward sympathetic cooling principle makes
them applicable to a huge variety of different species. Notwithstanding this
success, detailed simulations of buffer gas cells are rare, and have never been
compared to experimental data in the regime of low to intermediate buffer gas
densities. Here, we present a numerical approach based on a trajectory
analysis, with molecules performing a random walk in the cell due to collisions
with a homogeneous buffer gas. This method can reproduce experimental flux and
velocity distributions of molecules emerging from the buffer gas cell for
varying buffer gas densities. This includes the strong decrease in molecule
output from the cell for increasing buffer gas density and the so-called
boosting effect, when molecules are accelerated by buffer-gas atoms after
leaving the cell. The simulations provide various insights which could
substantially improve buffer-gas cell design.
