Using core-hole reference states for calculating X-ray photoelectron and emission spectra. Issue 18 (28th April 2022)

• Record Type: Journal Article
• Authors: Dreuw, Andreas; Fransson, Thomas
• Is Part Of: Physical chemistry chemical physics. Volume 24:Issue 18(2022)
• Page Start: 11259
• Page End: 11267
• Publication Date: 2022-04-28
• Subjects: Chemistry, Physical and theoretical -- Periodicals; 541.3
• DOI: 10.1039/d2cp00584k
• Languages: English
• ISSNs: 1463-9076

Abstract

Abstract : A protocol for removing near-singularities in post-HF calculations of core-ionization energies and X-ray emission spectra is presented, enabling highly reliable calculations of such properties for large molecules and when using large basis sets. Abstract : For the calculation of core-ionization energies (IEs), X-ray photoelectron spectra (XPS), and X-ray emission spectra (XES), a commonly applied approach is to use non- Aufbau reference states with a core-hole as either final (IE and XPS) or initial (XES) state. However, such reference states can introduce numerical instabilities in post-HF methods, relating to the denominator of the energy corrections involved. This may become arbitrarily close to zero if a negative virtual orbital is present, e.g. a core-hole, leading to near-singularities. The resulting instabilities lead to severe convergence issues of the calculation schemes and, in addition, can strongly affect both energies and intensities, with oscillator strengths seen to reach values up to 4 × 10 7 . For the K -edge we propose freezing the highest-energy virtual orbitals which contribute to any denominator below a threshold of 0.1 Hartree. Stable and reliable spectra are then produced, with minimal influence due to freezing energetically high-lying virtual orbitals (typically removing <5% of the total number of MOs). The developed protocol is here tested for Møller–Plesset perturbation theory and for the algebraic diagrammatic construction scheme for the polarization propagator, and it is also relevant for coupled cluster theory and other related methods.