Ab-initio simulation of isolated screw dislocations in bcc Mo and Ta

The equilibrium core structure of isolated. (a/2) [111] screw dislocations are calculated using a first-principles pseudopotential plane-wave method within the local-density approximation of density functional theory. The long-range strain field of the dislocation is treated using a variation of the recently developed lattice Green's function boundary condition method. This flexible boundary method allows the dislocation to be contained in a very small simulation cell without compromising the fidelity of the final core configuration. Supercells of 168, 270 and 504 atoms are used to evaluate the local screw and edge displacements of the (a/2) [111] screw dislocation in Mo and Ta. These results are contrasted with previous results from atomistic and dipole array calculations. We rnd that the isolated screw dislocations are evenly spread on three conjugate (110) planes for both Mo and Ta. The twinning-antitwinning anisotropy is calculated in Mo by applying a pure glide stress on the (112) plane. In these simulations the dislocation always moves on a {110} plane. The lattice frictional stress to move a straight screw dislocation on the (112) plane in the antitwinning and twinning sense is estimated at 0.025 mu and 0.0125 mu respectively. This is in good agreement with parallel atomistic simulations and experimental measurements of the slip asymmetry.