  The Peierls stress of the a/2<110> screw dislocation belonging to the shuffle
set is calculated for silicon using density functional theory. We have checked
the effect of boundary conditions by using two models, the supercell method
where one considers a periodic array of dislocations, and the cluster method
where a single dislocation is embedded in a small cluster. The Peierls stress
is underestimated with the supercell and overestimated with the cluster. These
contributions have been calculated and the Peierls stress is determined in the
range between 2.4 x 10-2 and 2.8 x 10-2 eV {\AA}-3. When moving, the
dislocation follows the {111} plane going through a low energy metastable
configuration and never follows the 100 plane, which includes a higher energy
metastable core configuration.
