where z i is the atomic transition charge (ATC) of atom i in molecule a, and d i j is the distance between atom i and j, respectively. The sum runs over all pairs of atoms, in which one atom belongs to molecule a and the other to molecule b. Thus, variations in molecule type are captured by the numerators of the terms in the sum, and variations in relative orientation are captured by the denominators. As shown in Figure , which compares couplings calculated with the ATC method with those computed rigorously using the EET (electronic energy transfer) method, the ATC method is surprisingly accurate. While the ATC method only includes the Coulombic contribution of exciton coupling, most molecules in experimentally known crystals are far enough away from each other that the other contributions to coupling (such as exchange contributions and orbital overlap components) make an insignificant contribution. The ATC method enables the DFT component of the coupling calculation to be performed using isolated molecule structures, therefore enabling rapid calculation of coupling based on interatomic distances for arbitrary molecular pairs. In turn, the ATC method may allow us to work with single-molecular databases, rather than large dimer databases, for building machine-learned models of exciton coupling.