The origin for the large computed values turns out to be related to the excitonic interactions or, most specifically, their distance dependence. In this respect, we stress that the present study goes beyond previous studies, where only nearest-neighbor excitonic couplings were considered in the Hamiltonian . In the latter case (which would be very much representative for charge transport), the resulting diffusion coefficient drops to cm 2 /s, that is about 3 orders of magnitude smaller than the value obtained when holding the full interaction range. Figure portrays the evolution of the diffusion coefficient as the number of neighbors, and thus the range of excitonic couplings, increases.