We first construct a model for the study of SF in pentacene and tetracene single crystals. For this, we select the dimer of pentacene/tetracene molecules and include the ground and excited states of each molecule for the modeling. We understand that the electronic coupling in pentacene and tetracene crystals could result in the delocalization of electronic wave function within few molecules, which points that the dimer model might not capture the whole electronic quantum dynamics in the real system. But our previous work demonstrated the validity of dimer model to study PSF process in pentacene . Briefly, at any given time, the coherence length is limited to a few pentacene molecules due to uncorrelated bath fluctuations. The lack of long-lived electronic coherence and subsequent SF show that the delocalized exciton quickly collapses to a localised triplet-pair state with two electrons occupying adjacent pentacene molecules . The CT state is considered here as a singlet electronic state without electronic transition dipole. In addition, the correlated triplet-pair state is modeled as a singlet electronic state with lower excited energy. The total Hamiltonian can be written as H = H mol + H env . The molecular part of Hamiltonian is given as