Relativistic Core-Valence-Separated Molecular Mean-Field Exact-Two-Component Equation-of-Motion Coupled Cluster Theory: Applications to L-edge X-ray Absorption Spectroscopy

TL;DR

This paper introduces a relativistic, core-valence-separated EOM-CCSD method within the X2C framework for accurate L-edge X-ray absorption spectra of transition metal complexes, outperforming previous approaches.

Contribution

The paper develops a novel relativistic EOM-CCSD approach with core-valence separation using the X2C method, providing improved accuracy for core-excitation spectra.

Findings

Accurately predicts L-edge features including energy shifts and fine-structure splittings.

Outperforms perturbative spin--orbit and relativistic TDDFT methods.

Establishes the method as a robust tool for relativistic core-excitation spectroscopy.

Abstract

L-edge X-ray absorption spectra for first-row transition metal complexes are obtained from relativistic equation-of-motion singles and doubles coupled-cluster (EOM-CCSD) calculations that make use of the core-valence separation (CVS) scheme, with scalar and spin--orbit relativistic effects modeled within the molecular mean-field exact two-component (X2C) framework. By incorporating relativistic effects variationally at the Dirac--Coulomb--Breit (DCB) reference level, this method delivers accurate predictions of L-edge features, including energy shifts, intensity ratios, and fine-structure splittings, across a range of molecular systems. Benchmarking against perturbative spin--orbit treatments and relativistic TDDFT highlights the superior performance and robustness of the CVS-DCB-X2C-EOM-CCSD approach, including the reliability of basis set recontraction schemes. While limitations remain in describing high-density spectral regions, our results establish CVS-DCB-X2C-EOM-CCSD as a powerful and broadly applicable tool for relativistic core-excitation spectroscopy.