Generalized stacking-faults and screw-dislocation core-structure in bcc iron: A comparison between ab initio calculations and empirical potentials

Generalized stacking fault energies and 1/2 < 111 > screw dislocation core structures are reported for two sets of models for iron: density functional theory (DFT) calculations and empirical potentials. A thorough comparison between various DFT approaches has been performed on {110} and {211} gamma-lines, which give a first indication on dislocation properties: (i) the effect of the exchange-correlation functional, LDA versus GGA, is significant in the pseudopotential approximation but not in the PAW approximation or in paramagnetic calculations; and (ii) the discrepancy due to the basis set between SIESTA and plane-wave results is rather small. Three empirical potentials for iron have been benchmarked on these DFT results. They all yield similar energies, but different shapes for the gamma-lines. Using the criterion suggested by Duesbery and Vitek, the gamma-line results point to non-degenerate core structures for the DFT calculations and for the Ackland and Ackland-Mendelev potentials but not for the Dudarev-Derlet potential. The direct calculations of the dislocation core structures show that the Ackland potential is an exception to the Duesbery-Vitek rule. More insight into the stability of the core structure can be gained by looking at the response to the polarization of the core. The Dudarev-Derlet and Ackland potentials have similar polarizations, but the energy difference between degenerate and non-degenerate cores is much larger with the Dudarev-Derlet potential, as expected from the gamma-lines. The polarizability of the non-degenerate core is smaller with the Ackland-Mendelev potential than in DFT, indicating that the energy landscape is flatter in this direction.