Ab Initio Multiplet-Plus-Cumulant Approach for Correlation Effects in X-Ray Photoelectron Spectroscopy.

Area of Science:

• Condensed matter theory
• Quantum chemistry
• Materials science

Background:

• Treating electronic correlations in open-shell systems is a major challenge in condensed matter theory.
• Existing methods like ligand-field multiplet theory and cumulant expansion have limitations, neglecting either atomic multiplets or itinerant states, respectively.
• Computationally intensive methods like dynamic mean-field theory are often required for more complete descriptions.

Purpose of the Study:

• To develop a more comprehensive and computationally feasible approach for electronic correlations in open-shell systems.
• To integrate the strengths of ligand-field multiplet theory and the cumulant expansion.
• To accurately model both local atomic multiplet effects and satellite excitations.

Main Methods:

• Separating dynamic Coulomb interactions into local and longer-range components using ab initio parameters.
• Combining ligand-field multiplet theory with the cumulant expansion for the one-electron Green's function.
• Applying the new approach to analyze experimental X-ray photoelectron spectroscopy (XPS) data.

Main Results:

• The combined multiplet-plus-cumulant approach successfully accounts for both local atomic multiplets and satellite excitations.
• The method accurately explains multiplet peaks, charge-transfer satellites, and distributed background features observed in XPS spectra.
• The approach provides a more complete description of electronic correlations in transition metal oxides.

Conclusions:

• The developed combined approach offers a powerful tool for understanding electronic correlations in complex materials.
• This method bridges the gap between atomic and itinerant electronic correlation descriptions.
• It provides a computationally efficient yet accurate way to interpret spectroscopic data for open-shell systems.