Computer Simulation of Photochemical Reactions Yusheng Dou, Department of Physical Sciences, Nicholls State University, Thibodaux, LA 70301 Photoinduced Formation of Cyclobutane Pyrimidine Dimers In the reaction shown in left , the stacked thymine molecules form a cyclobutane pyrimidine dimer, which is the main course of photoinduced DNA damage, and the formation of two chemical bonds linking two thymines occurs nonsynchronously after the excimer decays to electronic ground state. In the reaction shown in right, only one bond is formed between two thymine molecules. However, the bond breaks about 50 fs after formation and then two molecules move away from each other. This reaction leads to the DNA damage repair.