First, we compare the overall averaged energies ( E) between 1 S 0 and 3 T 1 along the central TTTT stack obtained from five replicas for each of the thymine in the 1T model. These energies range between 288 and 346 kJ.mol 1 along the 20 trajectories, and are subject to important fluctuations for the same QM part. Comparison of the singlet-to-triplet energies E suggests that the formation of the triplet state may be more favorable on T9 and T10 than on T8 and T7. Indeed, localization of spin density on the latter bases would lead to an energy penalty of 18 kJ.mol 1 . However, the di↵erence is not systematic along the trajectories, and falls in the range of the accuracy of our energy estimates. Conversely, for a given QM region, the variation are significant and may reach up to 43 kJ.mol 1 . Hence, one can hypothesize a scenario in which photosensitization, and triplet energy transfer, is possible due to dynamic and vibrational fluctuations leading to a temporary favorable alignment of the energy levels. Note also that a consistent, although moderate, stabilization of the triplet energy compared to the one of isolated thymine is observed already at this level of theory. Despite having access to the entire QM/MM-MD trajectories, the identification of the structural factors breaking the symmetry of the system and leading to the preference for the localization of the triplet on T9 and T10, are not self evident. Indeed, one has to take into account that the average values mask rather important deviations and oscillations and hence, should not be overinterpreted. Furthermore, the high variability of the energy value is also due to a subtle coupling between the DNA vibrational degrees of freedom, the solvent reorganization, and the counterions movements. As a matter of fact, ions have been recognized as important for triggering DNA chemistry and photochemistry and in many instance they play a non-innocent role. However, the detailed analysis of the ensemble of these factors, and their cross-talks, goes beyond the scope of the present contribution, and would probably require longer sampling time and sophisticated, or machine-learningbased, analytical tools. Furthermore, as shown in Supplementary Information, the analysis of DNA structural parameters, performed using Curves+ code, does not pinpoint any clear di↵erences between the behavior of the four thymines. Indeed, the Watson-Crick network is mostly conserved, in agreement with previous studies that have reported that the 1 S 0 !