  The ion-bombardment induced nanopatterning of Si(001) has been simulated by
atomistic simulations with and without Fe impurity. The surface contamination
has been simulated by using a new force field developed for FeSi. This is a
fitted bond order potential (BOP) given for Si and for Fe by Albe et al. This
BOP formula has been optimized simultaneously for FeSi, Si and for Fe. Using
this new force field we are able to follow the ion-beam assisted deposition for
Si in the presence of Fe contamination in the surface region. As an overall
result, we get an unexpectedly rich variety of nanopatterns formed by the
reorganization of the crater rims of the individual ion impacts. The previously
thought simple atomistically roughened surfaces show unprecedented landscapes
and topography with nanoscale features. The characteristic size of the units of
the pattern is in the range of a few nms. Typical of the occurred pattern is
the network of interconnected elongated adatom islands. We also see the
self-organization of this pattern upon ion-bombardments. At $50^{\circ}$ impact
angle we get a nanoporous surface (sponge-like) both for Fe-contaminated and
Fe-free simulations. At $70^{\circ}$ of impact (grazing angle of incidence) the
pattern resembles to that of elongated atomic chains (adatom islands) along the
ion impact direction. This latter pattern could be understood as a prepattern
state towards rippling. At lower angles ($30^{\circ}$) nanoholes rule the
landsdcape. The obtained pattern corresponds to low fluence experiments which
are used to consider as simple roughening without showing any sing of
patterning.
