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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Accuracy of approximate methods for the calculation of absorption-type linear spectra with a complex system-bath
J A Nöthling1, Tomáš Mančal2, T P J Krüger1
1Department of Physics, University of Pretoria, 0002 Pretoria, South Africa.
The Journal of Chemical Physics
|September 8, 2022
Summary
The Full Cumulant Expansion (FCE) method accurately calculates linear optical spectra for plant light-harvesting complexes. Other methods like Redfield approximations show limitations with strong pigment coupling.
Area of Science:
- Quantum Biology
- Spectroscopy
- Computational Chemistry
Background:
- Accurate calculation of linear optical spectra is crucial for understanding photosynthetic light-harvesting complexes.
- Approximate methods are widely used but their accuracy varies with system parameters.
Purpose of the Study:
- To evaluate the accuracy of approximate methods for calculating linear optical spectra in plant light-harvesting complexes.
- To determine the impact of energy gap and interpigment coupling on spectral calculation accuracy.
Main Methods:
- Comparison of Full Cumulant Expansion (FCE), coherent time-dependent Redfield (ctR), and Redfield/modified Redfield methods against exact stochastic path integral evaluation.
- Modeling a pigment dimer with chlorophyll a intramolecular modes and inhomogeneous disorder.
Main Results:
- FCE demonstrated the highest accuracy across all calculated spectra (absorption, linear dichroism, circular dichroism).
- ctR provided qualitative accuracy for moderate coupling but deviated significantly at strong coupling (>100 cm⁻¹).
- Redfield methods performed less accurately than ctR, especially with strong coupling or small energy gaps.
Conclusions:
- FCE is the most reliable approximate method for linear optical spectra in these systems.
- The accuracy of ctR and Redfield methods is sensitive to interpigment coupling strength and energy gap.
- Intramolecular modes and disorder did not significantly affect spectral accuracy under typical conditions.
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