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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
Published on: May 27, 2020
Exciton coupling mechanisms analyzed with subsystem TDDFT: direct vs pseudo exchange effects
Carolin König1, Johannes Neugebauer
1Theoretische Organische Chemie, Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstrasse 40, 48149 Münster, Germany.
The Journal of Physical Chemistry. B
|March 27, 2013
Summary
Exact exchange in hybrid functionals influences exciton splitting not through direct Dexter coupling, but by enhancing Coulomb coupling via modified transition densities. This finding aids in calculating excited states for large pigment systems.
Area of Science:
- Quantum Chemistry
- Computational Spectroscopy
- Materials Science
Background:
- Exciton coupling, crucial for energy transfer in molecular systems, is typically dominated by Coulombic interactions between localized excitations.
- Dexter-type exchange interactions are proposed for short-distance exciton coupling but are poorly modeled by standard time-dependent density functional theory (TDDFT) functionals.
- The impact of exact exchange incorporation in functionals on exciton splitting is significant but not fully understood.
Purpose of the Study:
- To elucidate the precise mechanism by which exact exchange influences exciton coupling and splitting.
- To differentiate between direct and indirect effects of exact exchange on exciton coupling using subsystem TDDFT.
- To investigate the role of Coulomb versus Dexter coupling in the presence of varying amounts of exact exchange.
Main Methods:
- Utilizing subsystem TDDFT to analyze exciton coupling mechanisms and distinguish direct/indirect effects.
- Performing calculations on 2-pyridone and chlorophyll dimers to model exciton interactions.
- Investigating the impact of varying percentages of exact exchange in hybrid functionals on electronic structure and excitation energies.
Main Results:
- The strong influence of exact exchange on exciton splitting arises from an enhanced Coulomb (pseudo-exchange) coupling, not a direct Dexter-type exchange interaction.
- Changes in transition densities, driven by exact exchange, are the primary cause of altered exciton coupling.
- Subsystem TDDFT successfully disentangled the contributions of Coulomb and exchange interactions to exciton coupling.
Conclusions:
- The effect of exact exchange on exciton splitting is predominantly mediated by Coulombic interactions, modulated by changes in transition densities.
- This work clarifies the fundamental nature of exciton coupling under hybrid functionals.
- A pathway for efficient excited-state calculations of large pigment aggregates using hybrid functionals is proposed, overcoming previous computational limitations.
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