Caterina Cocchi (Humboldt-Universität zu Berlin)
From Molecules to Organic Crystals: Optical Excitations from First Principles
Time-dependent (TD) density functional theory (DFT) is doing remarkably well in capturing the strongly bound excitons of isolated systems, even with the most simple exchange-correlation kernels, like the TD local density approximation (LDA). Though, due to a spurious long-range behavior, TD-LDA is known to fail in describing bound excitons in solids. Aiming at understanding when and why TD-DFT can be relied on, we compare this methodology with many-body perturbation theory (MBPT). To do so, we apply the most accurate approach to describe optical excitations, including the GW method and the Bethe-Salpeter equation. We systematically go from isolated molecules to their crystalline phase and perform an in-depth analysis of their absorption spectra, showing what governs the excitations when going from the molecule to the solid. In our study, we consider different systems, starting from small carbon-based molecules (e.g. methane) up to molecular materials such as oligothiophenes and azobenzenes, which are relevant for optoelectronic applications. Moreover, we analyze the influence of intermolecular orientation and interaction on the excitonic effects in these systems . All the calculations are performed with the all-electron code “exciting” (http://exciting-code.org), where all the quantities entering the TD-DFT and MBPT formalisms are treated on the same footing .
 C. Cocchi and C. Draxl, in preparation (2014)
 S. Sagmeister and C. Ambrosch-Draxl, Phys. Chem. Chem. Phys. 11, 4451 (2009)