Suryanarayanan Chandrasekaran

• Photosynthesis is one of the key processes to sustain life on earth. The initial step of photosynthesis involves capturing the sunlight by pigments in so-called light-harvesting complexes and transferring the excitation energy towards the reaction center where charge separation processes take place. In subsequent steps, the respective energy is used for the production of ATP. The transfer efficiency of the excitation energy to the reaction center might be enhanced by quantum effects. The detailed mechanism of this quantum effects is still under debate. In addition, this kind of quantum coherence effects might also help to improve the efficiency of (organic) solar cells. Sun light is not only used by plants as primary source of energy production but also bacteria and algae. In this thesis light-harvesting complexes from bacteria and algae are investigated theoretically. To this end, a multi-scale approach is employed using classical molecular dynamics simulation with subsequent electronic structure calculations and quantum dynamics. The Fenna-Matthews-Olson (FMO) complex of green sulfur bacteria and the Phycoerythrin 545 (PE545) antenna of marine algae are studied in detail. To be able to perform molecular dynamics simulations, one needs to obtain the respective force fields. Thus, here comparison between two different force fields for a bacteriochlorophyll molecule is carried out. As a test the so-called spectral density was determined, which describes the energy-dependent coupling between pigment and environment. Furthermore by calculating the excitonic coupling among the pigments, the population dynamics was determined using an ensemble-averaged wave packet formalism. In additon, a new parametrization of the bacteriochlorophyll a molecule was performed using the CGENFF formalism. Finally, the light-havesting complex PE555 has been simulated and compared to similar PE545 aggregate.