Using wavefunction and density-based ab initio approaches, we have evaluated the structural parameters and the electronic absorption spectra of three para disubstituted trans azobenzene derivatives: 4,4'-Cl, 4,4'-NO2, and 4,4'-NMe2. These calculations were performed in both gas-phase and ethanol, and large basis set effects have been unravelled at the second-order Moller-Plesset level. All three derivatives are found to present a planar azobenzene core. (c) 2007 Elsevier B.V. All rights reserved.