Further, in the middle row of Fig. we see that the excitation energy of the brightest transition monotonically rises with the number of molecules, with an exception of d = 3 Å. The excitation energy rises more strongly for shorter nearest neighbour distances d, as expected from PDM. However, quantitatively the first-principles excitation energies are smaller than those predicted with PDM (compare TD-DFT/CIS and PDM curves for d = 5.5 Å in Fig. , middle row). For d = 3 Å the excitation energy curves go down below the shown energy window since there is no bright state within the first calculated 50 states, starting at N = 7 for TD-ωB97X-D and at N = 9 for CIS, as can be seen in the lower row of Fig. where the oscillator strength of the brightest transition is shown. There we also see that for fixed N the oscillator strength is larger for larger d. It is opposite to expectation from the simple exciton model. In the latter, the oscillator strength is given (in atomic units) by