Configuration Interaction in Frontier Molecular Orbital\nBasis for Screening the Spin-Correlated, Spatially Separated Triplet\nPair State <sup>1</sup>(T···T) Formation

In the theoretical\nscreening of Singlet Fission rates in molecular\naggregates, often the frontier molecular orbital model for dimers\nis employed. However, the dimer approach fails to account for recent\nexperimental findings that suggest singlet fission progresses through\na further intermediate state featuring two spatially separated, spin-correlated\ntriplets, specifically a <sup>1</sup>(T···T) state.\nWe address this limitation by generalizing the often used frontier\nmolecular orbital model for singlet fission by incorporation of both\nseparated Charge Transfer (C···T) and <sup>1</sup>(T···T)\nstates as well as mixed triplet-charge transfer states, delivering\nanalytic expressions for the diabatic matrix elements. Applying the\nmethodology to the perylene diimide trimer, we examine the packing\ndependence of competing formation pathways of the <sup>1</sup>(T···T)\nstate by evaluation of diabatic matrix elements.