Photosystem II (PSII) catalyses light-driven water oxidation, supplying molecular oxygen and electrons for photosynthesis. The mechanism of oxygen-oxygen bond formation and the protonation states of the catalytic intermediates remain central questions in understanding this process. This project uses hybrid quantum mechanics/molecular mechanics (QM/MM) calculations to investigate how the local structure, electronic state and protein environment of the oxygen-evolving manganese-calcium cluster influence water oxidation. Atomic models derived from cryo-electron microscopy and time-resolved X-ray free-electron laser structures will provide the starting points for the calculations. The catalytic metal cluster, nearby substrate waters and selected residues will be treated using density functional theory, while the surrounding protein, membrane and solvent will be represented by molecular mechanics. TURBOMOLE will provide the quantum-mechanical energies and forces within a NAMD-based QM/MM molecular dynamics workflow. The requested allocation will support a focused set of calculations within my postdoctoral research at Stockholm University: local refinement of candidate catalytic structures, comparison of selected protonation and spin-state models, and short QM/MM trajectories at selected configurations relevant to oxygen-oxygen bond formation. These calculations will help identify models suitable for further sampling and assess their consistency with experimental structural data. The expected outputs are refined active-site models, comparative energetic and structural information, and validated starting configurations for the broader mechanistic study. This Small allocation will complement the existing group resources supporting the larger simulation programme. The results will contribute to understanding biological water oxidation and the catalytic principles underlying photosynthetic energy conversion.