Theory and Simulation of the Environmental Effects on FMO Electronic Transitions

Long-lived quantum coherence has been experimentally observed in the Fenna-Matthews-Olson (FMO) light-harvesting complex. It is much debated which role thermal effects play and if the observed low-temperature behavior arises also at physiological temperatures. To contribute to this debate, we use molecular dynamics simulations to study the coupling between the protein environment and the vertical excitation energies of individual bacteriochlorophyll molecules in the FMO complex of the green sulfur bacterium Chlorobaculum tepidum. The so-called spectral densities, which account for the environmental influence on the excited-state dynamics, are determined from temporal autocorrelation functions of the energy gaps between ground and first excited states of the individual pigments. Although the overall shape of the spectral density is found to be rather similar for all pigments, variations in their magnitude can be seen. Differences between the spectral densities for the pigments of the FMO monomer and FMO trimer are also presented.