| Abstract
| - An effective Frenkel-exciton Hamiltonian for the entire LH2 photosynthetic complex (B800, B850, andcarotenoids) from Rhodospirillum molischianum is calculated by combining the crystal structure with theCollective Electronic Oscillators (CEO) algorithm for optical response. Electronic couplings among all pigmentsare computed for the isolated complex and in a dielectric medium, whereby the protein environmentcontributions are incorporated using the Self-Consistent Reaction Field approach. The absorption spectra areanalyzed by computing the electronic structure of the bacteriochlorophylls and carotenoids forming the complex.Interchromophore electronic couplings are then calculated using both a spectroscopic approach, which derivescouplings from Davydov's splittings in the dimer spectra, and an electrostatic approach, which directly computesthe Coulomb integrals between transition densities of each chromophore. A comparison of the couplingsobtained using these two methods allows for the separation of the electrostatic (Förster) and electron exchange(Dexter) contributions. The significant impact of solvation on intermolecular interactions reflects the needfor properly incorporating the protein environment in accurate computations of electronic couplings. TheFörster incoherent energy transfer rates among the weakly coupled B800−B800, B800−B850, Lyc-B850,and Lyc-B850 molecules are calculated, and the effects of the dielectric medium on the LH2 light-harvestingfunction are analyzed and discussed.
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