We first examine the ππ * absorption band, corresponding to the bright, S 0 → S 2 transition of the monomer. The spectra of the monomer and the tetramer calculated at the single geometries (with the monomer geometry optimized at the B3LYP/def2-SV(P) level) are shown in Fig. . As can be expected for an H-aggregate, we observe a blue shift in comparison with the monomer spectrum, ∼0.24 eV in our case. The brightest transition in the tetramer is the S 0 → S 13 transition. [We note that the tetramer stick spectrum turned out to be rather sensitive to the monomer geometry-see spectra of tetramers constructed from the B3LYP/def2-SV(P) and B3LYP/def2-TZVP 26 optimized monomer geometries in Fig. and FTDM matrices in Fig. .] Using the simplest exciton model (which includes only local excitations and assumes large energy gaps between monomeric states), one would expect the brighest transition to be S 0 → S 8 . However, at the small intermolecular separation (3.5 Å) and with the ωB97X-D functional, we observe that transitions from S 0 → S 8 to S 0 → S 11 originate from the S 0 → S 3/4 monomeric transition, and the S 0 → S 12 transition is a charge-resonance 31 transition corresponding to the monomeric S 0 → S 2 transition. We also note that the S 0 → S 6 transition in the tetramer at 3.47 eV has a small but nonzero oscillator strength of ∼0.01. This transition is red-shifted by ∼0.64 eV with respect to the peak of the monomeric ππ * band.