QM/MM-MD simulations were performed using the Orca 36 suite of programs, interfaced with the Amber18 package. The QM partition was selected to encompass either one of the four thymine nucleobases (T7, T8, T9 or T10) or the whole four stacked thymines in order to assess the delocalization character of the triplet state (see Figure ). The QM subsystem consists in the nucleobases, with a link atom placed on the dangling N-glycosidic bond. QM/MM-MD trajectories were run for a total of 1 ns with a timestep of 1.0 fs. The resulting QM/MM and QM-only energies have been averaged over the last 500 ps. Note that 5 replicas have been run to increase the statistical sampling. Our chosen global time length is meaningful to provide a good analysis of the behavior of the triplet state in DNA, which can reach the ns scale. The singlet and triplet states have been obtained at density functional theory (DFT) and its time dependent (TD-DFT) extension level using the M062X functional and the Pople's double-⇣ 6-31G(d,p) basis set. This choice may, indeed, provide both an accurate representation of the electronic structure and a fast enough convergence allowing for a reasonable sampling of the phase space. The centroid of the spin density C S has been obtained as the integral over all the space of the product of the x, y, z coordinates with the spin density