Equation-of-Motion Coupled-Cluster Cumulant Green's Function for Excited States and X-Ray Spectra

Area of Science:

• * Computational quantum chemistry
• * Many-body theory
• * Spectroscopy

Background:

• * Green's function methods are essential for treating electron correlation in excited states and spectra.
• * Conventional methods often rely on the GW self-energy approximation, which has limitations in molecular systems.
• * Existing techniques may not fully capture non-linear effects crucial for accurate spectral predictions.

Purpose of the Study:

• * To extend Green's function methods for accurate calculation of molecular X-ray spectra.
• * To incorporate non-linear corrections and coupled-cluster excitations into the Green's function formalism.
• * To improve the prediction of X-ray photoemission spectra (XPS) and X-ray absorption spectra (XAS).

Main Methods:

• * Development of a non-perturbative, real-time coupled-cluster cumulant Green's function approach.
• * Solving a system of coupled first-order, non-linear differential equations for the cumulant.
• * Extension to include full coupled-cluster-singles (CCS) core Green's function with all single, double, and triple cluster excitations.

Main Results:

• * The method accurately yields spectral functions for core Green's functions, directly applicable to XPS.
• * Achieved excellent agreement for binding energies and satellite excitations.
• * Calculated XAS by convolving the core spectral function with an effective one-body XAS, including edge singularity corrections.

Conclusions:

• * The enhanced Green's function approach provides a robust framework for molecular spectroscopy.
• * The inclusion of non-linear cumulant contributions and CCS excitations significantly improves spectral accuracy.
• * The method is successfully demonstrated for the XPS and XAS of ammonia (NH3).