Figure displays the computed couplings in comparison with the dipole approximation for the displacement along the z axis. In this case, the S 0 →S 1 transitions are in a head-totail configuration, whereas the S 0 →S 2 transitions are in a stacked configuration. Therefore, in Figure we observe the expected J-type (negative coupling, panel (a)) and H-type (positive coupling, panel (b)) behavior (see also the respective eigenvectors in the Supporting Information, Table ). Overall the quality of the dipole approximation deteriorates with decreasing distance, i.e., starting from around 3-4 Å. For such short distances, however, the neglect of exchange is no longer justified as shown in Ref. 16. Finally, notice that the agreement between the two approaches is generally not as good for the S 0 →S 1 transitions.