Directional versus central-force bonding in studies of the structure and glide of 1/2 < 111 > screw dislocations in bcc transition metals

In this paper, we address the differences between Finnis-Sinclair potentials and bond-order potentials (BOPs) when studying 1/2 < 111 > screw dislocations in bcc transition metals, specifically Mo and W. These two types of potentials differ in that the former is central-force, whereas the latter include angular bonding. The cores of 1/2 < 111 > screw dislocations have two variants, one invariant with respect to the < 101 > diad and the other not. Hence, the latter core is degenerate. The BOPs always lead to the invariant type, whereas for Finnis-Sinclair potentials both variants occur. However, the symmetry of the core does not play a decisive role in the glide of dislocations. It is the description of interatomic forces that governs both the core structure and the glide behaviour of dislocations. The general characteristics of dislocation glide, the twinning-antitwinning asymmetry and a lower Peierls stress for tension than compression are the same for both types of potentials. Whereas the results obtained with BOPs are similar for the two cases studied, Finnis-Sinclair potentials lead to a broader variety. Particularly, the slip plane at 0 K is always {110} for BOPs but it is either {110} or {112} for Finnis-Sinclair potentials. The reason is that, in the latter case, the core configuration and core transformations are less constrained than in the former case. Hence, in bcc transition metals the BOPs are a more reliable description of atomic interactions than Finnis-Sinclair potentials, but when the d band does not play any important role, the Finnis-Sinclair potentials are fully applicable.