Prize Talk: Justin Jankunas Doctoral Dissertation Award Finalists: Developing a Quantum Chemical Toolbox for Accurate Modeling of K-edges and Beyond

X-ray spectroscopy is a vibrant field that has received considerable attention in recent years due to advances in light sources. By probing the electronic structure of inner-shell orbitals, it enables the investigation of local chemical environments through element specific spectral signatures. From a theoretical perspective, these core-excited states also pose some challenges to existing quantum chemical methods, especially due to lack of relaxation effects that arise after creation of a core-hole. Moreover, as we explore elements of the periodic table with higher atomic numbers, relativistic effects become more relevant, especially for the prediction of K (1s) and L (2p) shell transitions.

In this talk, we discuss different approaches to model X-ray emission (XES) and absorption (XAS), as well as the role of scalar relativistic effects in accurately modeling the core-excitation energies and the spectra of heavy elements. In the context of XES, we analyze the different ingredients that are necessary to accurately model oscillator strengths, and propose a simple metric that provides insights into some of the issues of the traditional linear-response methods, such as time-dependent density functional theory (TDDFT), to model core-spectroscopy.

For XAS, we combine the exact two-component one electron model (X2C) for relativistic effects with two DFT based approaches to study K-edge X-Ray spectroscopies for elements of the third row and early transition metals of the periodic table. The first approach is based on optimizing an excited state configuration directly, within the framework known as orbital-optimized (OO) DFT. The second approach, electron-affinity TDDFT (EA-TDDFT), relies on a double linear-response on top of a core-ionized reference, and attempts to fix the shortcomings of regular TDDFT for core-spectroscopy. Both OODFT and EA-TDDFT, when combined with X2C, show remarkable agreement in predicting K-edge X-Ray absorption (XAS) spectra of a wide range of molecules containing third row main group elements, with RMSE < 0.5 eV. We also investigate how higher order relativistic effects, such as retardation and vacuum fluctuations, play a significant role in the core binding energies of transition metals and how this affects the accuracy of our proposed methods.