Abstract: Photosynthesis is a particularly attractive target for the design of potential commercial herbicides. Since photosynthesis is a mechanism that occurs in plants only, all inhibitors are in theory nontoxic to mammals. A large number of inhibitors, including ureas, triazines and phenols etc., are known to block photosystem Ⅱ by displacing plastoquinone from the Q_B-binding niche of the D1 protein in the reaction center. The X-ray study by Deisenhofer et al. of the photosystem of Rps viridis was an important milestone, since it initiated molecular modeling studies in academia and instustry in order to find new PS Ⅱ inhibitors.