Real-Time Equation-of-Motion CCSD Cumulant Green's Function.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Simulating many-body excitations in X-ray photoemission spectra (XPS) from first principles is computationally challenging.
• Previous work established a cumulant-based one-electron Green's function method using the real-time coupled-cluster-singles equation-of-motion approach (RT-EOM-CCS).

Purpose of the Study:

• To extend the RT-EOM-CCS method to incorporate double excitations for improved accuracy in simulating XPS.
• To investigate the impact of including double excitations on ground-state energy and cluster amplitudes.

Main Methods:

• Implementation of double excitations within the RT-EOM-CCS framework using the Tensor Contraction Engine (TCE).
• Application of the extended real-time coupled-cluster singles and doubles equation-of-motion (RT-EOM-CCSD) approach to core-hole spectral functions.
• Utilizing core-optimized basis sets for small molecular systems.

Main Results:

• The inclusion of doubles contributions significantly reduces mean absolute errors in core binding energies for 10-electron systems (from 0.8 to 0.3 eV).
• The RT-EOM-CCSD method substantially improves the quasiparticle-satellite gap, reducing overestimation from 3-5 eV to 0-1 eV for CH4, NH3, and H2O.
• The method enhances the overall shape of satellite features in XPS spectra.

Conclusions:

• The extended RT-EOM-CCSD method provides a more accurate, nonperturbative cumulant form of the Green's function.
• The inclusion of double excitations is crucial for accurate simulation of core-level XPS spectra.
• The developed approach, even with the singles approximation and a modest basis set, is effective for studying carbon speciation in larger molecules.