Modeling of excitation dynamics in photosynthetic light-harvesting complexes: exact versus perturbative approaches

We compare various theoretical approaches that are frequently used for modeling the excitation dynamics in photosynthetic light-harvesting complexes. As an example, we calculate the dynamics in the major light-harvesting complex from higher plants using the standard Redfield theory, the coherent modified Redfield theory combined with the generalized Förster theory, and the scaled hierarchical equation of motion (HEOM). The modified Redfield and coherent modified Redfield theories predict unrealistically fast transfers between weakly coupled and isoenergetic sites due to the secular character of these approaches. This shortcoming can be excluded by the artificial breaking of exciton mixing between these sites and invoking a generalized Förster theory to calculate the transfers between them. A critical cutoff indicating which exciton couplings should be broken is dependent on the energy gap between the corresponding sites (and therefore can be different for different parts of the complex). An adequate determination of the strongly coupled compartments of the whole complex allows us to obtain a quantitatively correct description with the combined Redfield-Förster approach, resulting in kinetics not much different from the exact HEOM solution.