An azobenzene molecule Disperse Red 1 with strongly delocalised frontier orbitals has been studied with a number of density-functional theory (DFT) related approaches. The purpose is two-fold: to interpret observed photoabsorption and to compare the performance of various DFT-based approximations. The planarity of the vacuum conformation of the lower energy trans conformation is found to be significantly distorted when transformed to the higher energy cis conformation, and both isomers are found to absorb in the experimentally observed wavelengths in solutions. A common feature in both is essentially forbidden lowest energy absorption because of a negligible overlap of HOMO and LUMO due to symmetry unmatch. We find the time-dependent DFT to be the best approach for quantitative evaluation of photoabsorption energies and intensities, whereas Kohn-Sham eigenenergies and orbitals give good description only qualitatively. Isomerisation and dimerisation energetics have also been evaluated, and trans dimers have found to be up to 1 eV per molecule more stable than single molecules.