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The 8-oxo-deoxyguanosine is the most abundant specie of the DNA oxidative damage. Despite the deleterious effects such as gene mutation it may cause, the 8-oxodG was also reported to have beneficial effect such as repairing the nearby cyclobutane pyrimidine dimer (CPD) after photoexcitation. Due to its strong biological relevance, the photoinduced excited state dynamics behavior of 8-oxo-deoxyguanosine is of particular interest. In this work, a theoretical investigation by combination of complete active space self-consistent field (CASSCF) ab initio calculations and on-the-fly nonadiabatic dynamics simulations are implemented to provide intrinsic deactivation mechanism of its free base 8-oxoguanine after being excited to the S 1 and S 2 states. Two minimum energy conical intersections (MECIs) characterized by the C3-puckered motion with attractive chiral character are located, which contribute appreciably to the S 1 state deactivation process. When the system is being excited to the S 2 state directly, a S 2 → S 1 → S 0 two-step decay pattern is proposed. A nearly planar S 2 /S 1 intersection plays a significant role in the S 2 → S 1 decay process. The subsequent S 1 state relaxation process is also dominated by the C3-puckered deformation motion. One decay time is estimated to be 704 fs, which compares well with the experimental observation of 0.9 ± 0.1 ps in solvents. Particular illustration is the fact that the MECIs configurations we located bear an exceptional resemblance with previous reported thymine, cytosine and guanine, suggesting that the current work could lend support for better understanding of the non-natural nucleobases and derivatives. Unlabelled Image • Provide a comprehensive picture of the deactivation mechanism of the 8-oxo-G system on the S 1 and S 2 states. • Provide support for better understanding of the photochemical and photophysical properties of non-natural nucleobases. • Provide reasonable theoretical explanation to the experimental observations of the 8-oxo-G system. [ABSTRACT FROM AUTHOR]