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Assessment of Excited-State Methods for One- and Two-Photon Absorption in the Retinal Protonated Schiff Base
Duccio Di Prima1, Peter Reinholdt1, Jacob Kongsted1
1Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, DK-5230 Odense M, Denmark.
The Journal of Physical Chemistry. A
|May 26, 2026
Summary
Computational methods accurately predict optical properties of retinal protonated Schiff base (rPSB), crucial for designing light-sensitive rhodopsins for optogenetics and biomedical research.
Area of Science:
- Computational chemistry
- Photochemistry
- Biophysics
Background:
- The retinal protonated Schiff base (rPSB) is the core chromophore in rhodopsins, essential for light-sensitive protein functions.
- Accurate prediction of rPSB optical properties aids in designing novel rhodopsin variants for optogenetics and biomedical applications.
- While complex methods are needed for isomerization, cost-effective approaches can assess vertical absorption properties.
Purpose of the Study:
- To benchmark computational methods for predicting vertical absorption and two-photon absorption (TPA) properties of rPSB.
- To evaluate the accuracy of ISR-ADC(2), LR/QR-CC2, and range-separated TDDFT functionals against EOM-EE-CCSD.
- To assess performance across different basis sets (aug-cc-pVDZ, 6-31+G*) and configurational space.
Main Methods:
- Benchmarking of vertical excitation energies (ΔEabs), oscillator strengths, and TPA transition strengths (δTPA) for the S0-S1 transition.
- Comparison of ISR-ADC(2), LR/QR-CC2, and various TDDFT functionals (e.g., CAM-B3LYP) against EOM-EE-CCSD.
- Utilizing aug-cc-pVDZ and 6-31+G* basis sets for calculations.
Main Results:
- ISR-ADC(2) demonstrated strong agreement with EOM-EE-CCSD for all evaluated optical properties.
- LR/QR-CC2 showed moderate agreement, with notable discrepancies in TPA strengths.
- TDDFT methods accurately reproduced excitation energies and oscillator strengths; functionals with long-range exchange corrections correlated well with EOM-EE-CCSD for TPA but underestimated values by ~4x.
Conclusions:
- ISR-ADC(2) is a reliable and cost-effective method for predicting rPSB optical properties.
- TDDFT methods, particularly those with tuned long-range exchange, show promise for TPA calculations, though systematic underestimation requires attention.
- Tuning DFT exchange parameters can improve agreement with higher-level methods like EOM-EE-CCSD or LR/QR-CC2.
