Open Access

Carsten Olbrich

• Photosynthesis is certainly one of the most important processes of energy conversion on earth. Plants, algae and some type of bacteria harvest the light energy and convert it into chemical energy. Subsequent to the initial absorption of light, which induces an excitation in a pigment molecule, the absorbed energy is transferred to the so-called reaction center (RC) and induces a charge separation and finally a synthesis of ATP. The transport of the absorbed light energy to the RC happen with almost 100̃\% quantum efficiency. To this end, certain quantum effects seem to play an important role but are not yet completely understood, for example the recently experimentally observed long-lived quantum coherences in the energy transport. The availability of high-resolution X-ray crystal structures of pigment-protein complexes provides the opportunity to investigate the excitation energy transfer and the related quantum effects on the atomic level. To that purpose, this thesis comprises a multi-scale approach using classical molecular dynamics simulations with subsequently applied electronic structure calculations to compute optical properties and excited state dynamics of light-harvesting (LH) systems without additional parameters. This multi-scale approach was applied to two LH systems: the LH2 complex of a purple bacterium and the FMO complex of a green-sulfur bacterium. Consecutive excitation energies for the individual pigments and the electronic couplings between the pigments were obtained for each of the two complexes. These quantities were combined to a time-dependent Hamiltonian which were subsequently used to calculate optical properties and excitation energy transfer dynamics. Based on the fluctuations of the excited state energies, spectral densities were obtained which serve as necessary input quantities in dissipative quantum dynamics calculations. Additionally, spatial correlations of excitation energies, couplings and atomic motions were determined. Such correlations have been proposed as possible origin of the long-lived quantum coherences in the energy transport but found to be very small.