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With the advent of improved synchrotron-based X-ray sources, core-level spectroscopy has secured its
position as one of the most lucrative avenues for probing the electronic structure of materials as well as
for fingerprinting various complex chemical phenomena. Evidently, dedicated theory-efforts are needed
to complement the experimental research. At the same time, leveraging the rapidly expanding database
of experimental spectra, computational core-level spectroscopy can offer valuable insights on the methods
and approximations employed in electronic structure theory and thereby inspire the formulation of new
theoretical approaches. Following a brief “teaser” report on X-ray absorption to demonstrate the utility of
simulations, in this talk I will explore the efficacy and interrelation of two popular first-principles frameworks
in the context of simulating core-level emission spectroscopy. For non-resonant X-ray emission spectroscopy
(NXES), I will show that a constrained-occupation based optimized-orbital Kohn-Sham treatment is more
reliable than the adiabatic linear-response (LR) approach. The inadequacy of the latter is, in fact, rooted
in the neglect of dynamical electron-hole screening, which can be associated with the change in polarization
resulting from the core-ionization. I will demonstrate that an inexpensive determinant-based formalism
reliant only on two self-consistent field calculations is typically sufficient for modeling NXES. Equipped
with this understanding, we can develop a flexible framework for simulating the resonant inelastic X-ray
scattering (RIXS) process by treating the effects of the core-excitations within the constrained-occupation
approach and valence excitations with the LR method. Such a division of treatment not only ensures fast and
accurate simulation of the RIXS spectrum, but also helps with its interpretation by automatically assigning
the spectral features to the relevant electronic transitions. In a pseudopotential-based implementation, this
approach circumvents the need for all-electron calculations. This framework can also be straightforwardly
extended to incorporate the effects of additional excitations like vibrations, charge-transfer, etc.
References
• S. Roychoudhury, L. A. Cunha, M. Head-Gordon, and D. Prendergast
Changes in polarization dictate necessary approximations for modeling electronic de-excitation inten-
sity: an application to x-ray emission ,
Physical Review B 106, 075133 (2022).
• S. Roychoudhury and D. Prendergast
CleaRIXS: A Fast and Accurate First-Principles Method for Simulation and Analysis of Resonant
Inelastic X-ray Scattering ,
Physical Review B 106, 115115 (2022).
• S. Roychoudhury and D. Prendergast
Efficient core-excited state orbital perspective on calculating X-ray absorption transitions in determi-
nant framework,
arXiv:2208.11261 (2022). |