Geometry dependence of excitonic couplings and the consequences for configuration-space sampling

Area of Science:

• Computational Chemistry
• Materials Science
• Quantum Mechanics

Background:

• Excitonic coupling is crucial for understanding excitonic energy transport (EET) in systems like organic photovoltaics.
• Accurate wavefunction methods are computationally expensive for realistic systems, necessitating lower-scaling semi-empirical methods for EET modeling.

Purpose of the Study:

• To evaluate the distance and angle dependence of excitonic couplings for organic molecule dimers using the LC-DFTB and SOS-CC2 methods.
• To assess the accuracy and computational efficiency of LC-DFTB for modeling excitonic couplings compared to DFT and wavefunction methods.

Main Methods:

• Semi-empirical long-range corrected density functional based tight binding (LC-DFTB) method.
• Spin opposite scaled second order approximate coupled cluster singles and doubles (SOS-CC2) method.
• Analysis of excitonic couplings and their dependence on dimer geometry.

Main Results:

• LC-DFTB accurately reproduces the excitonic couplings and their dependence on dimer geometry.
• LC-DFTB achieves accuracy comparable to DFT (ωB97X) but with significantly reduced computation time (several orders of magnitude faster).
• Analysis of exchange contributions to excitonic couplings was performed, enabling efficient calculation of Coulombic excitonic couplings.

Conclusions:

• LC-DFTB is a viable and computationally efficient method for modeling excitonic couplings in organic systems.
• This work provides a foundation for simulating excitonic energy transport using semi-empirical methods.
• The findings facilitate the study of EET in complex organic materials like those used in photovoltaics.