Theoretical X-Ray Spectroscopy of Transition Metal Compounds. (arXiv:1812.08822v2 [physics.chem-ph] UPDATED)

X-ray spectroscopy is one of the most powerful tools to access structure and
properties of matter in different states of aggregation as it allows to trace
atomic and molecular energy levels in course of various physical and chemical
processes. X-ray spectroscopic techniques probe the local electronic structure
of a particular atom in its environment, in contrast to UV/Vis spectroscopy,
where transitions generally occur between delocalized molecular orbitals.
Complementary information is provided by using a combination of different
absorption, emission, scattering as well as photo- and autoionization X-ray
methods. However, interpretation of the complex experimental spectra and
verification of experimental hypotheses is a non-trivial task and powerful
first principles theoretical approaches that allow for a systematic
investigation of a broad class of systems are needed. Focussing on transition
metal compounds, L-edge spectra are of particular relevance as they probe the
frontier d-orbitals involved in metal-ligand bonding. Here, near-degeneracy
effects in combination with spin-orbit coupling lead to a complicated multiplet
energy level structure, which poses a serious challenge to quantum chemical
methods. MCSCF theory has been shown to be capable of providing a rather
detailed understanding of experimental X-ray spectroscopy. However, it cannot
be considered as a 'blackbox' tool and its application requires not only a
command of formal theoretical aspects, but also a broad knowledge of already
existing applications. Both aspects are covered in this overview.

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