Because the total nuclear relaxation or the polaronic energy is larger than the excitonic couplings, solving the dynamics with this Hamiltonian leads to thermalized excitons that are essentially sitting on single polymer chains (with a calculated mean inverse participation ratio, 〈 〉 , of at room temperature). Yet, subspace surface hopping non-adiabatic molecular dynamics simulations (methodology in detail is given in the supplemental material) using this Hamiltonian and naturally including transient delocalization of the singlet excitons yield a diffusion coefficient, , of 0.23 cm 2 /s approaching the experimentally measured value of cm 2 /s .