Berlin 2012 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 71: Poster Session: Graphene / Topological Insulators / Interfaces and Surfaces

HL 71.23: Poster

Wednesday, March 28, 2012, 16:00–19:00, Poster D

Analysis and Improvement of the modified Becke-Johnson exchange potential — •David Koller, Fabien Tran, and Peter Blaha — Institute of Materials Chemistry, Vienna University of Technology, A-1060, Vienna, Austria

The modified Becke-Johnson exchange potential [1] (TB-mBJ) is the sum of an approximation to Slater’s averaged exchange potential and a response contribution, with the weight of both parts determined by the parameter c=c(∇ρ/ρ). It is a semilocal potential which leads to surprisingly good energy gaps for semiconductors and insulators. Its predictive power is better than that of hybrid-DFT calculations and similar to the much more expensive GW approach. The improvement is explained by the different spatial distribution of electrons in the valence band maximum and conduction band minimum states. For correlated TM compounds TB-mBJ works similarly to LDA+U and improves magnetic moments, band gaps and electric field gradients but (correctly) does not shift the occupied states below the O-p-band.

Although the performance of TB-mBJ is impressive, some cases would be described even better by a different choice of c. Therefore we investigated three strategies for improvement: reparametrization of the relation between c and (∇ρ/ρ), determining c from a different quantity and using a position-dependent c(r). The first strategy turns out to be helpful, especially when grouping the systems into categories like sp-semiconductors, wide band gap insulators or (non-)magnetic transition metal compounds.