  Motivated by the recent investigations on instabilities caused by Schwoebel
barriers during growth and their effects on growth or sublimation by step
flows, we have investigated, using the Stillinger-Weber potential, how this
step edge barrier arises for the two high symmetry steps on 1$\times$1
reconstructed Si(111). Relative to a barrier of 0.97 $\pm$ 0.07 eV on the
surface, we find additional (Schwoebel) barriers of 0.61 $\pm$ 0.07 eV and 0.16
$\pm$ 0.07 eV for adatom migration over the [$\overline{2}11$] and the
[$\overline{1}\overline{1}2$] steps respectively. The adatom potential energy
is found to be strongly correlated with that derived from the local geometry of
atoms on the adatom-free surface or step edges. This correlation preserves a
strict correspondence between the barrier determining features in the spatial
variation of the adatom potential energy and the same derived from the local
geometry for the Si(111) surface and the [$\overline{2}11$] step. It is
therefore argued that the Schwoebel barrier on the [$\overline{2}11$] step is
robust {\it i.e.} a feature that would survive in more satisfactory {\it ab
initio} or tight binding calculations. Using a diffusion equation for the
adatom concentration the relevance of the barrier to electromigration of steps
has been explored. Data from such experiments on Si(111) has been used to place
an upper bound on the Schwoebel barrier and a lower bound on the
electromigration force.
