Pooja Sarngadharan

• Photosynthesis is an essential process through which sunlight is converted into chemical energy, sustaining virtually all life on Earth. In the specific case of oxygenic photosynthesis, Photosystem II (PSII) plays a pivotal role as a major component of the photosynthetic machinery, responsible for the initial light absorption and generating molecular oxygen. This process involves numerous protein-pigment complexes within PSII. The photosynthetic apparatus is rich in colored pigments, which not only make it visually appealing but also crucial for capturing and transporting sunlight through excitation energy transfer. Due to the large size of these proteins and the electronic complexity of the pigment molecules embedded in the membrane, multiscale quantum-classical methods are essential for studying the processes. The protein environment significantly influences the tuning of the excitation energy of the pigments, thereby establishing an energy funnel in such systems. This thesis aims to deepen our understanding of the lightharvesting process in the antenna complexes of PSII. To achieve this, a multiscale approach is employed. This involves using the density functional tight-binding (DFTB) method to perform ground state molecular dynamics within a quantum mechanics/molecular mechanics (QM/MM) framework, coupled to an electrostatic classical environment. Following this, the time-dependent extension of the long-range-corrected DFTB is applied to obtain the excitation energies of each pigment molecule, within a QM/MM setting. This method generates essential excitonic parameters such as site energies, couplings, and spectral densities, which are utilized to model the spectroscopic properties. Furthermore, the calculated results have been compared with experimental data, showing great agreement for the antenna complexes in PSII. This alignment ensures the robustness of the methods, validating their use for studying light harvesting in both plant and cyanobacterial systems.