Exciton Hamiltonian for the bacteriochlorophyll system in the LH2 antenna complex of purple bacteria

An effective Frenkel-exciton Hamiltonian for the LH2 photosynthetic complex from Rhodospirillum molischianum is calculated using the collective electr onic oscillator (CEO) approach combined with the crystal structure. The abs orption spectra of the various bacteriochlorophyll aggregates forming the c omplex are computed using the CEO. Each electronic transition is further an alyzed in terms of its characteristic electron-hole motions in real space. Using a two-dimensional representation of the underlying transition density matrices, we identify localized and delocalized electronic transitions, te st the applicability of the exciton model, and compute interchromophore ele ctronic couplings. Forster energy-transfer hopping time scales within B800 and from the B800 to the B850 system, obtained using the computed coupling constants, are in excellent agreement with experiment.