Comparison of multireference ab initio wavefunction methodologies for X-ray absorption edges: A case study on [Fe(II/III)Cl4]2-/1- molecules

Dimitrios Maganas; Joanna K Kowalska; Marcel Nooijen; Serena DeBeer; Frank Neese

doi:10.1063/1.5051613

Comparison of multireference ab initio wavefunction methodologies for X-ray absorption edges: A case study on [Fe(II/III)Cl₄]^2-/1- molecules

J Chem Phys. 2019 Mar 14;150(10):104106. doi: 10.1063/1.5051613.

Authors

Dimitrios Maganas¹, Joanna K Kowalska², Marcel Nooijen³, Serena DeBeer², Frank Neese¹

Affiliations

¹ Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.
² Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
³ Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.

PMID: 30876345
DOI: 10.1063/1.5051613

Abstract

In this work, we present a detailed comparison of wavefunction-based multireference (MR) techniques for the prediction of transition metal L-edge X-ray absorption spectroscopy (XAS) using [Fe(II)Cl₄]^2- and [Fe(III)Cl₄]^1- complexes as prototypical test cases. We focus on the comparison of MR Configuration Interaction (MRCI) and MR Equation of Motion Coupled Cluster (MREOM-CC) methods, which are employed to calculate valence excitation as well as core to valence Fe L-edge XAS spectra of [Fe(II)Cl₄]^2- and [Fe(III)Cl₄]^1- complexes. The two investigated approaches are thoroughly analyzed with respect to their information content regarding (1) metal-ligand covalency, (2) ligand field splittings, (3) relativistic effects, (4) electron correlation, (5) energy distribution, and (6) intensity modulation of the experimentally observed spectral features. It is shown that at the level of MRCI calculations in both [Fe(II)Cl₄]^2- and [Fe(III)Cl₄]^1- cases, very good agreement with the experimental Fe L-edge XAS spectra is obtained provided that the employed active space is extended to include ligand-based orbitals in addition to metal-based molecular orbitals. It is shown that this is necessary in order to correctly describe the important σ- and π- Fe-Cl covalent interactions. By contrast, MREOM-CC calculations yield excellent agreement relative to experiment even with small active spaces. The efficiency of the employed MR computational protocols is thoroughly discussed. Overall, we believe that this study serves as an important reference for future developments and applications of MR methods in the field of X-Ray spectroscopy.