  Owing to their non-planar cores 1/2<111> screw dislocations govern the
plastic deformation of BCC metals. Atomistic studies of the glide of these
dislocations at 0 K have been performed using Bond Order Potentials for
molybdenum and tungsten that account for the mixed metallic and covalent
bonding in transition metals. When applying pure shear stress in the slip
direction it displays significant twinning-antitwinning asymmetry for
molybdenum but not for tungsten. However, for tensile/compressive loading the
Schmid law breaks down in both metals, principally due to the effect of shear
stresses perpendicular to the slip direction that alter the dislocation core.
Recognition of this phenomenon forms a basis for the development of physically
based yield criteria that capture the breakdown of the Schmid law in BCC
metals. Moreover, dislocation glide may be preferred on {110} planes other than
the most highly stressed one, which is reminiscent of the anomalous slip
observed in many BCC metals.
