Stephan Sagmeister1,2 and Claudia Ambrosch-Draxl1,
1Chair of Atomistic Modelling and Design of Materials, University of Leoben, Austria
2Materials Center Leoben, Austria
In this work we address excitonic effects in the macroscopic dielectric function of variuos band-gap materials. To this extent we pursue two different routes, which are many-body perturbation theory (MBPT) by solving the Bethe–Salpeter equation (BSE), and time-dependent density functional theory (TDDFT). Since both approaches are implemented in the same code, i.e., all-electron full-potential, linearized augmented planewave (LAPW) package exciting, all the underlying quantities are treated on the same footing, hence allowing for the best possible comparison of the applied methodologies. We explore the performance of various exchange-correlation kernels like the ALDA, model kernels and the BSE-derived kernel. We show results for simple sp semiconductors  and insulators as well as for polymers and organic molecular crystals. For the latter, the transition from an isolated molecule to the bulk phase is discussed by means of the short-range ALDA kernel and a comparison to the BSE.
 S. Sagmeister and C. Ambrosch-Draxl, Phys. Chem. Chem. Phys. 11, 4451 (2009)
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