The simulation of X-ray absorption spectra from ground and excited electronic states using core-valence separated DFT/MRCI

Issaka Seidu; Simon P Neville; Martin Kleinschmidt; Adrian Heil; Christel M Marian; Michael S Schuurman

doi:10.1063/1.5110418

The simulation of X-ray absorption spectra from ground and excited electronic states using core-valence separated DFT/MRCI

J Chem Phys. 2019 Oct 14;151(14):144104. doi: 10.1063/1.5110418.

Authors

Issaka Seidu¹, Simon P Neville¹, Martin Kleinschmidt², Adrian Heil², Christel M Marian², Michael S Schuurman¹

Affiliations

¹ Department of Chemistry and Biomolecular Sciences, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada.
² Institute of Theoretical and Computational Chemistry, Heinrich Heine Universität Düsselddorf, Universitätsstraße 1, 40225 Düsseldorf, Germany.

PMID: 31615239
DOI: 10.1063/1.5110418

Abstract

We present an extension of the combined density functional theory (DFT) and multireference configuration interaction (MRCI) method (DFT/MRCI) [S. Grimme and M. Waletzke, J. Chem. Phys. 111, 5645 (1999)] for the calculation of core-excited states based on the core-valence separation (CVS) approximation. The resulting method, CVS-DFT/MRCI, is validated via the simulation of the K-edge X-ray absorption spectra of 40 organic chromophores, amino acids, and nucleobases, ranging in size from CO₂ to tryptophan. Overall, the CVS-DFT/MRCI method is found to yield accurate X-ray absorption spectra (XAS), with consistent errors in peak positions of ∼2.5-3.5 eV. Additionally, we show that the CVS-DFT/MRCI method may be employed to simulate XAS from valence excited states and compare the simulated spectra to those computed using the established wave function-based approaches [ADC(2) and ADC(2)x]. In general, each of the methods yields excited state XAS spectra in qualitative and often quantitative agreement. In the instances where the methods differ, the CVS-DFT/MRCI simulations predict intensity for transitions for which the underlying electronic states are characterized by doubly excited configurations relative to the ground state configuration. Here, we aim to demonstrate that the CVS-DFT/MRCI approach occupies a specific niche among numerous other electronic structure methods in this area, offering the ability to treat initial states of arbitrary electronic character while maintaining a low computational cost and comparatively black box usage.