Since arg(J mn ) = 0 and π for J mn > 0 and J mn < 0, respectively, for a homodimer, ∆E = 0, all oscillator strength will be in the upper excited state if J mn > 0 (H-aggregate), whereas it will be in the lower state if J mn < 0 (J-aggregate). In the case of a heterodimer, ∆E ̸ = 0, both excited states carry oscillator strength and the monomer limit is recovered for ∆E/J mn ≫ 1. The two cases allow for a straightforward interpretation in the case of the dipole approximation. Here one speaks about head-to-tail (J-case) and stacked (H-case) configurations. Sliding the monomers of a stacked configuration relative to each other, one observes a transition from one type of aggregate to the other one at the magic angle of 54.7 • between d m and R mn . Note that in the case of the spatially extended transition densities, these simple considerations may be violated leading to a magic angle different from the simplistic dipole case, as it will be demonstrated in Sec. 3.2.