Core structure and Peierls potential of screw dislocations in alpha-Fe from first principles: cluster versus dipole approaches

A comparison between the dipole and cluster approaches for the study of the core structure and Peierls potential of 1/2 < 111 > screw dislocations in bcc metals is presented. It is based on first principles electronic structure calculations in alpha iron carried out within the DFF framework using localized basis functions as implemented in the SIESTA code. The effect of the energetic model is first investigated on the {211} and {110} generalized stacking fault energy (gamma) surfaces which are known to be closely related to the dislocation core properties. All DFT results yield similar shapes-characteristic of a non-degenerate core structure-and the effect of the exchange-correlation functional is shown to be larger than the discrepancies between SIESTA and planewave-pseudopotential results. The core structure is found to be non-degenerate, with an excellent agreement between the various approaches on the deviation from the linear elasticity theory of the atomic positions. In the dipole approach, the interaction between dislocations is dominated by elastic effects, but significant anisotropic core-core interactions are evidenced, which strongly affect the energetics of the system when a triangular array of dipoles is used. For the calculation of the Peierls potential a very good agreement is obtained between the cluster approach and the dipole approach, provided that a quadrupolar-like arrangement is used. Similar calculations are performed with the EAM potential proposed by Mendelev et al. [Philos. Mag. 83, 3977 (2003)] for iron; the comparison between the two sets of results is briefly discussed.