Excitonic Hamiltonian for Singlet Fission: Beyond a Dimer Model

calculations on monomers and dimers of molecules and is highly suitable for the study of aggregates. This excitonic model is then used to solve for the energetics of large aggregates. This coarse-grained model is demonstrated specifically on the pentacene crystal, where we evaluated the spectra and density of states. We show the natural emergence of bands of states in some cases and characterize them. Through an analysis of the participation ratio of the eigenstates, we gain crucial insight into the extent of their multireference character. This method is useful in understanding not just the structure of these extended aggregates but also as a cornerstone for incorporation of vibronic features and simulation of the singlet fission dynamics at a quantum-classical or semiclassical level.