Efficient tool to calculate two-dimensional optical spectra for photoactive molecular complexes.

Area of Science:

• Quantum dynamics
• Spectroscopy
• Theoretical chemistry

Background:

• Photoactive molecular complexes play crucial roles in natural processes like photosynthesis.
• Accurate theoretical modeling of their dynamics is essential for understanding energy transfer.
• Existing methods have limitations in describing system-environment interactions.

Purpose of the Study:

• To develop and validate a combined theoretical approach for calculating two-dimensional photon-echo spectra.
• To assess the accuracy of the combined method against numerically exact calculations.
• To evaluate its applicability for simulating natural photosynthetic complexes.

Main Methods:

• Combining coherent modified Redfield theory (CMRT) with the equation of motion-phase matching approach (PMA).
• Utilizing the quasi-adiabatic path integral (QAPI) approach for numerically exact comparisons.
• Calculating two-dimensional photon-echo spectra for the Fenna-Matthews-Olson complex and a dimer model.

Main Results:

• CMRT accurately describes population dynamics decay rates but shows slight differences in stationary populations and oscillation frequencies compared to QAPI.
• The combined CMRT+PMA method shows excellent agreement with QAPI for coherent dynamics at short waiting times.
• For longer waiting times and strong system-bath coupling, differences arise due to distinct stationary states.

Conclusions:

• The combined CMRT+PMA approach provides reasonable results with acceptable computational cost for weak to intermediate system-bath couplings.
• This method is particularly suitable for simulating natural photosynthetic complexes.
• The study validates a computationally efficient tool for quantum dynamics simulations.