Ab initio spectroscopy is a powerful combination of quantum-based theories and computer simulations, covering a wide range of theoretical and computational methods that incorporate many-body effects and interactions showing up in the excited state. This framework, combining density-functional theory (DFT) with many-body perturbation theory, not only allows for analyzing data obtained by experimental probes, but also for shining light onto the underlying physics. I will demonstrate this with a series of selected materials and excitation processes:
Oxygen K-edge spectra from the wide-gap semiconductor Ga2O3 will reveal how signals from atoms located in a particular environment can be selectively enhanced or quenched by adjusting the crystal orientation . These results suggests ELNES, combined with ab initio many-body theory, to be a very powerful technique to characterize complex systems, with sensitivity to individual atomic species and their local environment.
With the example of self-assembled phases of functionalized azo-benzene it will be shown, how excitonic effects in core and valence excitations can be tuned by molecular packing , and how this may affect the switching functionality of the molecules.
Finally, I will discuss how transmission electron microscopy can be used for mapping atomic orbitals, exploring its capabilities by a first principles approach . For defected graphene, exhibiting either an isolated vacancy or a substitutional nitrogen atom, different kinds of images are to be expected, depending on the orbital character (see Figure 1).
 C. Cocchi, H. Zschiesche, D. Nabok, A. Mogilatenko, M. Albrecht, Z. Galazka, H. Kirmse, C. Draxl, and C. T. Koch, submitted to Phys. Rev. B (2016).
 C. Cocchi and C. Draxl, Phys. Rev. B 92, 205105 (2015).
 L. Pardini, S. Löffler, G. Biddau, R. Hambach, U. Kaiser, C. Draxl, and P. Schattschneider, Phys. Rev. Lett. (2016); in print.
To cite this abstract:Claudia Draxl; From core and valence excitations to orbital mapping: a theorist’s perspective. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/from-core-and-valence-excitations-to-orbital-mapping-a-theorists-perspective/. Accessed: May 26, 2020
EMC Abstracts - https://emc-proceedings.com/abstract/from-core-and-valence-excitations-to-orbital-mapping-a-theorists-perspective/