While we have clearly underlined the role of DNA, and most specific the e↵ects of ⇡ stacked chromophores in modulating the energy alignment between singlet and triplet states, the question of a localized or delocalized state, as well as of spin density mobility, remains unanswered. To tackle this issue, we report the spin density calculated over equidistant snapshots extracted from the QM/MM-MD simulation. An illustration of some representative snapshots is also provided in Figure , while the corresponding movie can be seen in Supplementary Information. It appears evident that the spin density is, for the majority of the time, highly localized over only one thymine nucleobase. Also coherently with the results of the 1T model a marked preference for T9 is observed. Interestingly enough, while partial delocalization between two consecutive chromophores was observed only very rarely, it is clear that ⇡ stacking is mostly acting by enhancing polarization rather than by inducing long-range spread excitonic states. If the spin density is localized it is nonetheless extremely mobile, indeed during the time spanned by our QM/MM-MD we may observe the population of virtually all the thymine nucleobases. Furthermore, the triplet transfer seams to proceed via a non-coherent hop-based mechanism rather than by coherent energy transfer, which should require persistent delocalized states. To assess more quantitatively the localization of the spin density, we report in Figure the evolution of the position of its centroid C S projected on the DNA vertical axis. We can firstly point out a coherent behavior between the di↵erent replicas, again evidencing a highly localized spin-density. Indeed, in most of the cases C S is localized in close proximity to the center of mass of T9, the small deviation presented may also be seen as an evidence of spin polarization. All the five replicas also agree in evidencing a high preference for spin localiza-tion on the central thymines rather than at the periphery. This result, which may appear in disagreement with the 1T model can however be easily rationalized considering the border e↵ect of the QM/MM partition, which disfavour the peripheral chromophores. However and coherently with what observed from the representative movie we may still evidence a rather high spin mobility for all the replicas and especially for trajectory 4 and 5. Indeed, while we can see that the spin density may localize on virtually all the thymines, we also evidence, especially for trajectory 4, the establishment, at around 800 ps, of an oscillatory behavior with the unpaired electrons rapidly swapping between T9 and T8. Interestingly, in this case the increased distance of C S with the center of mass of the corresponding base could also suggest a stronger amount of delocalization, or at least an increased polarization.