Simulating X-ray Absorption Spectra With Density Cumulant Theory
Ruojing Peng1 , Andreas V. Copan2 , and Alexander Yu. Sokolov∗1
1Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States
2Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
Near-edge X-ray absorption spectroscopy (NEXAS) is a powerful experimental technique that provides electronic structure information based on excitation of core electrons into low-lying unoccupied orbitals. In practice, NEXAS spectra often require inputs from theoretical simulations that facilitate their interpretation. However, since core-level excitations lie in the high-energy spectral region, NEXAS spectra are expensive to simulate using traditional excited-state methods, such as equation-of-motion coupled cluster (EOM-CC), algebraic diagrammatic construction (ADC), and time-dependent density functional theory (TD-DFT). Furthermore, accurate simulation of NEXAS spectra requires balanced description of electron correlation in both ground and excited states, as well as using diffused basis sets. One technique that allows direct access to the core-excitation region is the core-valence separation (CVS) approximation. In the CVS approximation, excitations from core and valence occupied orbitals are decoupled, thereby tremendously reducing the excitation manifold of interest. In this work, we develop an efficient approach for simulating NEXAS spectra by applying the CVS approximation to linear response density cumulant theory (LR-DCT) [1]. We assess the accuracy of CVS-approximated LR-DCT (CVS-LR-DCT) against full LR-DCT and benchmark its performance for a set of small molecules by computing their spectra and comparing with experimental results.