  Using dark matter only N-body cosmological simulations, we measure the
pericentre distance of dark matter halos on their first infall into group and
cluster halos. We find that the pericentre distance (R$_{\rm{peri}}$) is an
important parameter as it significantly affects the strength of tidal mass loss
in dense environments, and likely other environmental mechanisms as well. We
examine what determines the R$_{\rm{peri}}$ value and find that, for most
infallers, the dominant parameter is V$_{\rm{\perp}}$, the tangential component
of the orbital velocity as the halo enters the group/cluster halo for the first
time. This means that the strength of tidal stripping acting near the cores of
groups/clusters are strongly influenced by the external peculiar velocity field
of the large scale structure surrounding them, which differs between clusters,
and is sensitive to the mass ratio of infaller to host. We find that filament
feeding also partially contributes to feeding in low V$_{\rm{\perp}}$ halos.
Dynamical friction can also play a role in reducing R$_{\rm{peri}}$ but this is
only significant for those few relatively massive infallers ($>$10\% of the
mass of their host). These results highlight how the response of galaxies to
dense environments will sensitively depend on dynamics inherited from far
outside those dense environments.
