  We reexamine two essential issues within the Peierls-Nabarro model which are
critical in obtaining accurate values for the Peierls stress. The first issue
is related to the sampling scheme of the misfit energy across the glide plane
and the second one is the effect of relaxation on the Peierls stress. It is
shown that most of the current applications of the Peierls-Nabarro model do not
treat properly the two issues and therefore are not able to predict reliable
values for the Peierls stress. We argue that the double counting scheme for the
misfit energy at both sides of the glide plane is physically more reasonable,
and it can predict more accurate values for the Peierls stress, especially for
dislocations with equal spacing between alternating atomic rows. We also show
the importance for allowing atomic relaxation when a dislocation traverses the
Peierls barrier, which in turn lowers the Peierls stress for narrow
dislocations by an order of magnitude.
