Over the last decade, advancements made in the new generation of synchrotron facilities have enabled the production of high coherence, high brightness soft x-rays, which has dramatically increased the resolution of x-ray spectroscopy. X-ray absorption (XAS) and emission (XES) spectroscopic methods provide the ability to study the local electronic structure near the exited atom in complex systems. As various x-ray spectroscopies have become increasingly accurate, there is a growing need to be able to generate accurate spectra theoretically to help decipher experimental results. This necessitates the use of highly accurate electronic structure methods along with the incorporation of relativistic effects into the calculations. The K- and L-edge regions of x-ray spectra have been extensively studied and benchmarked using various theoretical methods including time-dependent density functional theory (TDDFT), equation-of-motion coupled-cluster (EOM-CC), and complete active space (CAS) methods. Here, we push the limits of these methods further and delve into the M-edge region for various lanthanide systems. Among other systems that include f-block elements, we discuss the M<sub>4,5</sub>-edge of cerium hexaboride, which corresponds to excitations from the 3d core orbitals on the cerium into unoccupied orbitals. By using TDDFT and restricted active space configuration interaction (RASCI) methods, we generate XAS spectra. Relativistic effects are included variationally into these calculations through use of the exact two-component (X2C) Hamiltonian. The resulting spectra are then compared to each other and to experimental XAS spectra and resonant inelastic x-ray scattering (RIXS) spectra to validate the accuracy of these methods.

Generating M-edge X-ray spectra for lanthanide compounds using relativistic post-SCF methods

ACS Spring 2022

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