Green’s function methods within many-body perturbation theory provide a general framework for treating electronic correlations in excited states and spectra. In an extension of their previous collaborative work, researchers at the University of Washington and Pacific Northwest National Laboratory developed a cumulant form of the one-electron Green’s function using a real-time coupled-cluster equation-of-motion approach. This approach yields a nonperturbative expression for the cumulant in terms of the solution to a set
of coupled first order, nonlinear differential equations, and thereby naturally adds nonlinear corrections to traditional cumulant methods.
The approach is applied to the core-hole Green’s function and illustrated for small molecular systems. For these systems, the nonlinear contributions are crucial, yielding accurate core-level binding energies (typically within 0.7 eV) and satellites in X-ray photoelectron spectroscopy. This approach can be systematically improved, including an exact CC-cumulant with double excitations and a more correlated reference.