  The fully two-dimensional Peierls barrier map of screw dislocations in BCC
iron has been calculated using the first principles method to identify the
migration path of a dislocation core. An efficient method to correct the effect
of the finite size cell used in the first principles method on the energy of a
lattice defect was devised to determine the accurate barrier profile. We find
that the migration path is close to a straight line which is confined in a
$\{110\}$ plane and the Peierls barrier profile is single-humped. This result
clarifies a reason that the existing empirical potentials of BCC iron fail to
predict the correct mobility path. A line tension model, incorporating these
first principles calculation results, is used to predict the kink activation
energy to be 0.73 eV in agreement with experiment.
