  The breakdown of the Schmid law in bcc metals has been known for a long time.
The asymmetry of shearing in the slip direction <111> in the positive and
negative sense, respectively, commonly identified with the
twinning-antitwinning asymmetry, is undoubtedly one of the reasons. However,
effect of stress components other than the shear stress in the slip direction
may be important. In this paper we investigate by atomistic modeling the effect
of shear stresses perpendicular to the Burgers vector on the glide of a/2<111>
screw dislocations. We show that these shear stresses can significantly elevate
or reduce the critical resolved shear stress (CRSS) in the direction of the
Burgers vector needed for the dislocation motion, i.e. the Peierls stress. This
occurs owing to the changes of the core induced by these stresses. This effect
may be the reason why slip systems with smaller Schmid factors may be preferred
over that with the largest Schmid factor.
