Quasiparticle Self-Consistent GW-Bethe-Salpeter Equation Calculations for Large Chromophoric Systems

Area of Science:

• Computational Chemistry
• Quantum Mechanics
• Spectroscopy

Background:

• The GW-Bethe-Salpeter equation (BSE) method is valuable for calculating molecular excitonic states.
• Current implementations are limited to small molecules and rely on mean-field starting points.
• A need exists for methods applicable to larger systems and independent of starting points.

Purpose of the Study:

• To implement and validate a self-consistent, starting-point-independent quasiparticle self-consistent (qsGW)-BSE approach for large molecular systems.
• To assess the accuracy of qsGW-BSE for calculating vertical excitation energies (VEEs) compared to experimental data and other methods.
• To investigate the electronic excitation properties of photosystem II (PSII) reaction center chromophores.

Main Methods:

• Development and implementation of the self-consistent qsGW-BSE method.
• Calculation of VEEs for chlorophyll monomers and dimers, comparing with experimental gas-phase spectra.
• Application of qsGW-BSE to calculate low-lying excitation energies of the six chromophores in the photosystem II reaction center.

Main Results:

• The qsGW-BSE method provides VEEs in excellent agreement with experimental spectroscopic data for chlorophyll systems.
• Eigenvalue-only self-consistency was found to be insufficient for describing certain excitonic states in chlorophyll dimers.
• Calculations on the PSII reaction center hexamer revealed predominantly local excitation characters in the low-energy spectrum, consistent with previous TD-DFT studies.
• The qsGW-BSE method enabled efficient calculations on large systems with thousands of correlated electrons and basis functions.

Conclusions:

• The developed qsGW-BSE approach is a robust and scalable method for accurate electronic excitation energy calculations in large molecular systems.
• qsGW-BSE offers a significant improvement over traditional methods, providing accurate results independent of the starting point.
• Future studies should incorporate environmental effects to fully capture charge-transfer phenomena observed in biological systems like photosystem II.