The FTDM matrices for the brightest transitions of the decamers with increasing d are shown in Fig. . First, the matrices are virtually diagonal, with some small nearest neighbour CT contributions at smaller d. Next, we see that while the exciton is delocalized over the decamer, the contributions of local excitations of central monomers are larger than those of the terminal molecules -this is the effect of the finite size of the chain. Furthermore, we see that the diagonal values change with d (the heat map normalization in Fig. is done with respect to the maximum element among all shown matrices). We note that it is not expected from the simple PDM exciton theory, in which the wave function coefficients c n (see above) do not change with varying separation of chromophores. The diagonal values F XX as a function of monomer X are plotted in Fig. , for TD-ωB97X-D and CIS calculations. The F XX curves become more concave with increasing d. This effect is more pronounced at the TD-ωB97X-D level than at the CIS level. The overall CT contributions (shown with dotted lines in Fig. ) are higher for TD-ωB97X-D. Namely, at the TD-ωB97X-D level they are 8, 4, 1, 0.3, 0.05 % for d = 3.5, 3.7, 4.0, 4.5, 5.5 Å, respectively. At the CIS level, the corresponding total CT contributions are 1.6 %, 1.1 %, 0.6 %, 0.2 %, 0.02 %. Having examined the TD-DFT/CIS results for the chains consisting of up to ten molecules, we turn now to the BSE@G 0 W 0 @PBE calculations. We were particularly interested in the absorption spectrum of an infinite (periodic) chain. But at first we performed BSE@G 0 W 0 @PBE calculations for the monomer. Here, two approaches were used -(i) BSE@G 0 W 0 @PBE calculations with numeric atom-centered orbitals as implemented in FHI-aims, and (ii) BSE@G 0 W 0 @PBE calculations with linearized augmented plane waves as implemented in exciting. The monomer spectra calculated using these two approaches are shown in orange in Fig. . The lowest energy absorption maxima are at 3.43 eV (FHI-aims) and 3.50 eV (exciting), so there is a remarkable agreement between the two approaches. We also note that these values are slightly smaller than the excitation energy calculated at the BSE@evGW @PBE level (evGW is an iterative GW method, in which the quasiparticle gap is converged within 0.1 eV), which is ∼3.7 eV. We also recall that the experimental excitation energy of the gas-phase monomer is about 4.1 eV, as mentioned above. The higher energy parts of the monomer spectra, however, show a larger discrepancy (compare orange curves in Fig. ), which we attribute to differences between the used approaches (molecular vs. periodic). We next calculated the spectra of the dimer and the trimer, both with d = 3.5 Å, using FHI-aims (Fig. , upper panel). The monomer-to-dimer blue shift is ∼0.21 eV and the monomer-to-trimer blue shift is ∼0.29 eV. These shifts are larger in comparison with those calculated with TD-DFT (∼0.13 eV and ∼0.20 eV, respectively, at the TD-ωB97X-D level) and with CIS (∼0.16 eV and ∼0.23 eV, respectively).