Furthermore, we computed the absorption spectrum of the infinite linear chain with d = 3.5 Å, using exciting (see Fig. , lower panel). Here, we observe a very pronounced monomer-to-chain blue shift, of ∼1.6 eV. This blue shift is much larger than the TD-DFT/CIS monomer-to-decamer blue shifts of about 0.3-0.4 eV (see Tab. 2). The strong chain peak at ∼5.1 eV is only ∼1.7 times intenser than the monomer peak at ∼3.5 eV. We note, however, that the spectrum is normalized to the unit cell in this case. We also note weak absorption (in the BSE@G 0 W 0 @PBE spectrum) at energies near and below the monomer peak, which, in principle, can also be used to excite the chain. : BSE@G 0 W 0 @PBE calculations. Upper panel: FHI-aims calculations for the monomer, dimer 3.5 Å, and trimer 3.5 Å. Lower panel: exciting calculations for the monomer and H-type chain 3.5 Å. Vertical grey line shows the experimental, gas-phase monomer peak position. A deeper analysis shows that the weak transition at ∼3.7 eV in the periodic chain arises mainly from the VBM-1 to CBM excitations at Γ and K. (Here, VBM stands for the valence-band maximum, and CBM for the conduction-band minimum.) The same states contribute to the monomer exciton at 3.5 eV. For the intense exciton transition at ∼5.1 eV (for the chain), apart from the dominant VBM-1 to CBM contribution, there is also a sizable contribution from VBM-1 to CBM+1 at K. The contributions of the bands to these excitons are shown in Fig. .