  In body-centred-cubic (bcc) metals migrating 1/2<111> screw dislocations
experience a periodic energy landscape with a triangular symmetry. Atomistic
simulations, such as those performed using the nudged-elastic-band (NEB)
method, generally predict a transition-pathway energy-barrier with a
double-hump; contradicting Ab Initio findings. Examining the trajectories
predicted by NEB for a particle in a Peierls energy landscape representative of
that obtained for a screw dislocation, reveals an unphysical anomaly caused by
the occurrence of monkey saddles in the landscape. The implications for motion
of screws with and without stress are discussed.
