Comparison of multireference ab initio wavefunction methodologies for X-ray absorption edges: A case study on...

Area of Science:

• Computational Chemistry
• Spectroscopy
• Quantum Chemistry

Background:

• Transition metal complexes are crucial in various chemical and biological processes.
• Accurate prediction of X-ray absorption spectroscopy (XAS) is vital for understanding electronic structures.
• Multireference (MR) methods are essential for accurately describing systems with strong electron correlation, common in transition metals.

Purpose of the Study:

• To compare the performance of wavefunction-based MR techniques, specifically MR Configuration Interaction (MRCI) and MR Equation of Motion Coupled Cluster (MREOM-CC).
• To evaluate these methods for predicting Fe L-edge X-ray absorption spectra (XAS) of tetrachloroiron complexes ([Fe(II)Cl4]2- and [Fe(III)Cl4]1-).
• To analyze the methods' ability to capture key spectral features including covalency, ligand field splittings, and electron correlation.

Main Methods:

• Detailed comparison of MRCI and MREOM-CC computational protocols.
• Calculation of valence and core-to-valence Fe L-edge XAS spectra for [Fe(II)Cl4]2- and [Fe(III)Cl4]1-.
• Analysis of spectral features: metal-ligand covalency, ligand field splittings, relativistic effects, electron correlation, energy distribution, and intensity.

Main Results:

• MRCI calculations achieve good agreement with experimental Fe L-edge XAS spectra when the active space includes ligand orbitals, essential for describing Fe-Cl covalent interactions.
• MREOM-CC calculations provide excellent agreement with experimental spectra even with smaller active spaces.
• Both methods' efficiency and information content regarding spectral features are thoroughly discussed.

Conclusions:

• MREOM-CC is a highly efficient and accurate method for predicting Fe L-edge XAS, performing well even with limited active spaces.
• MRCI requires larger active spaces including ligand orbitals to accurately reproduce experimental spectra, highlighting the importance of describing covalency.
• This study provides a valuable reference for selecting and applying MR methods in X-ray spectroscopy of transition metal complexes.