As a first step of our study we calculated absorption spectra of trans azobenzene using TD-DFT, TD-HF and CIS with the def2-SV(P) basis set. A moderate (double-zeta) basis set was chosen to enable direct comparison with calculations for large aggregates (see below), for which better basis sets (e.g. triple-zeta) are impractical. In addition, we tested several semiempirical methods. The calculated absorption spectra of the monomer are shown in Fig. . The lowest energy peak (for each method) corresponds to the S 0 → S 2 , ππ * transition. The S 0 → S 1 , nπ * transition is optically forbidden. The calculated ππ * absorption peak position ranges from 2.97 eV (TD-PM7) to 4.79 eV (CIS). The experimental, gas-phase value is about 4.1 eV. The ab initio CIS yields the most blue-shifted band (4.79 eV), followed by TD-HF (4.53). Notably, the difference between TD-HF and CIS is quite large, 0.26 eV, which may point to a ground-state RHF→UHF (triplet) instability. Indeed, the stability analysis confirmed the presence of the triplet instability on the HF/def2-SV(P) level, for the used monomer geometry (optimized with B3LYP/def2-TZVP). Further to the red follow the long-range corrected hybrids ωB97 (4.44 eV), M11 (4.39 eV), and ωB97X (4.35 eV), see spectra in shades of blue in Fig. . Next to them (to the red) go the global hybrid BHandHLYP, with 50 % exact exchange (4.21 eV), long-range corrected functional ωB97X-D (4.19 eV), and range-separated functional CAM-B3LYP (4.17 eV), see spectra in shades of green. These three methods yield the ππ * peak positions closest to the experimental value (4.1 eV). In this respect, we should note that we compare here only vertical excitation energies, neglecting vibrational fine structure. Further to the red follows the semiempirical TD-lc-DFTB method (3.90 eV) and global hybrid B3LYP, with 20 % of exact exchange (3.85 eV). Then follow semiempirical methods AM1/CIS (3.50 eV), TD-AM1 (3.44 eV), PM7/CIS (3.12 eV), and the most red-shifted TD-PM7 (2.97 eV). We also note that the oscillator strength of the S 0 → S 2 transition is much smaller for the used semiempirical methods (f < 0.5) than for TD-DFT, TD-HF, and ab initio CIS (f > 0.75).