Protein Effects on the Optical Spectrum of the Fenna-Matthews-Olson Complex from Fully Quantum Chemical Calculations.

We present a fully quantum-chemical study on the optical spectra of the Fenna-Matthews-Olson (FMO) protein. We have investigated the structural and environmental effects on the site energies and excitonic couplings as well as on the UV/vis absorption spectra. Our largest model of the entire protein-pigment network contains more than 7000 atoms. Structures of all bacteriochlorophyll pigments have been optimized in their binding pockets, comprising more than 1000 atoms in some cases. We find that the site energies are quite sensitive to structural and environmental changes in the model setup, while excitonic couplings are more robust. It is shown that nonoptimized pigment structures lead to site energies closer to the ones for pigments optimized in their binding pocket than to those for conformations fully optimized in vacuum. The determination of reliable site energies is one of the key factors for an understanding of the excited-state properties of the FMO protein.