On the Choice of Reference Orbitals for Linear-Response Calculations of Solution-Phase K-Edge X-Ray Absorption Spectra

02 September 2022, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


The simplest procedures for computing vertical excitation spectra in condensed-phase are configuration interaction with single excitations (CIS) and linear-response time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation. In applications to X-ray absorption spectroscopy (XAS), methods like CIS and TDDFT that codify only single excitations into the wave function are prone to catastrophic errors in main-edge and post-edge features whose shapes act as a crucial fingerprint in structural analyses of liquids. We show that these errors manifest primarily due to a lack of orbital relaxation in conventional linear-response theories and that core-ionized (n-1-electron) references, like those of electron-affinity TDDFT, can eliminate the errors in the spectral profile, even in the highest-energy parts of the post-edge. Crucially, we find that single excitations atop core-ionized references are sufficient to elucidate liquid-phase XAS spectra with semi-quantitative accuracy, opening the door for methods like electron-affinity CIS/TDDFT to be used as efficient alternatives to higher-order wave function approaches.


X-ray absorption spectroscopy
time-dependent density functional theory
configuration interaction singles
electron-affinity configuration interaction singles

Supplementary materials

Supplementary Information
Comparison between full EA-TDDFT solutions and those obtained from the Tamm-Dancoff approximation, all linear shifts applied to each spectrum, and spectra for each system represented as weighted histograms.


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