To arbitrate between previous conflicting theoretical and experimental results, a quantum-mechanical investigation of the ground-state geometry and UV spectrum of trans-azobenzene has been performed by using the Moller-Plesset approach and time-dependent density functional theory. It turns out that gas-phase Irans-azolbenzene is consistently predicted to be planar by the most accurate approaches. In solvent, the energy difference between planar and twisted trans-azobenzene is also negligible, although the twisted form tends to be favoured by a polar surrounding medium. These predictions are supported by comparisons between measured and predicted excitation energies. (c) 2005 Elsevier B.V. All rights reserved.