FIG. A. Illustration of the electronic energy transfer (EET) method of computing exciton couplings. The EET method involves computing a two-body integral over the transition density of a dimer. B. Illustration of the atomic transition charge (ATC) method. The ATC method involves computing a sum over pairs of transiton charges from the respective molecules. C. Coupling values as obtained using the EET and ATC methods for dimers from 1989 crystals. components. See Section IID for details.