We are investigating a new ab-initio approach to compute L-edge X-ray absorption spectra for strongly correlated systems such as iron-sulfur metal clusters. The new approach can consider important electron-electron couplings which give energy splittings in the L-edge spectra: multiplet splitting, crystal-field splitting, and spin-orbit splitting. We have investigated efficient ways to treat the electron-electron couplings based on matrix product states.<br/>X-ray absorption spectroscopy (XAS) is one of the key techniques for determining the local geometric and electronic structure of the metal clusters. The L-edge absorption causes an electronic excitation from 2p to the valence d of a metal atom. It is the electric-dipole allowed transition which has large intensity. Changes of oxidation state, spin state, and bond covalency lead to changes of intensity and energy splittings in the spectra. However, the interpretation of the L-edge spectra is extremely challenging.<br/>Traditionally, charge transfer multiplet theories have been used to simulate the L-edge spectra. More recently, various ab-initio methods have been used, including restricted open-shell configuration interaction singles (ROCIS) [1], time-dependent density functional theory (TDDFT) [2], restricted active space self-consistent field (RASSCF) [3], multi-reference configuration interaction (MRCI) [4], and multi-reference equation of motion coupled cluster (MR-EOMCC) [4]. However, ROCIS and TDDFT cannot describe multi-reference character in ground state for the metal clusters, while other multi-reference methods are difficult to apply to large active spaces (c.a. larger than 20 orbitals) which is essential for the metal clusters.<br/>The method we propose can address these shortcomings. Furthermore, the new method is easy to extend to the spectra of excited states as well. These attractive features of the new method open a new possibility to unveil electronic structure of the iron-sulfur metal clusters.

Computing L-edge X-ray absorption spectra of metal clusters with a new ab-initio approach based on matrix product states

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.