Excitation dynamics in photosynthetic light-harvesting complex B850: exact solution versus Redfield and Förster...

We model the excitation dynamics in bacterial B850 antenna using the standard Redfield, modified Redfield, and Förster approaches and comparing them with the exact solution obtained with hierarchical equation of motion (HEOM). We have found that the modified Redfield is capable of reproducing the dynamics associated with downhill relaxation from higher exciton levels, but fails to explain the migration of quasi-equilibrated excitation over the B850 ring (and its spectral signatures like transient anisotropy decay). Neglecting the population-to-coherence transfers leads to a quick decoherence between the exciton states resulting in unrealistically fast delocalization looking like instantaneous transfer around the ring. The standard (non-secular) Redfield gives a more satisfactory picture of this kind of migration, but in some cases the results can be corrupted by artifacts emerging from the one-phonon character of this theory.