A detailed study on the torsional potential of nitrobenzene is performed by using state-of-the-art ab initio methods, including density functional theory (DFT) ones. Special emphasis is given to basis set incompleteness in order to get complete agreement with experimental results. On the other hand, the DFT derived the energy difference between planar and perpendicular conformers is mostly dominated by the proportion of the HF exchange contribution to the exchange-correlation functional. A systematic application of hybrid schemes, energy decomposition analysis, and scan of the conjugative, steric, and weak interactions effects along the torsion will be used to rationalize the torsional profiles. (C) 2003 American Institute of Physics.