Figure shows how the coupling changes for a Q y -shift. As expected, at 0.0 Å Q y -shift, the coupling has a positive sign, and the system shows the characteristics of an H-aggregate since the two dipole vectors are perfectly stacked and aligned by construction. The large coupling value at 0.0 Å Q y -shift translates into a large energy difference between the excitonic excitation energies, as shown in the top panel of Figure . The Q y -shift causes the Coulomb coupling to approach zero, which is reached at a value of ca. 5.4 Å. On the one hand, as expected, a negligible coupling leads to nearly degenerate excitonic excitation energies (cf. Eq. ( )), which , on the other hand, are lower than the excitation energy of the monomer (top panel of Figure ). Since the two monomers are still relatively close, the interaction between locally non-balanced charge densities (matrix elements with A m = D m and/or B n = C n in Eq. ( )), i.e. permanent dipoles, causes a red-shift of the excitation energies. A further Q y -shift induces negative excitonic coupling values, which increase in absolute value along the shift and produce a J-aggregate. Passing from H-to J-aggregate can be observed also through the computed oscillator strength for excitonic excitations shown in the bottom panel of Figure . The consequences for a simulated absorption spectrum along Q y -shift are shown in Figure in the Supporting Information.