The potential energy surface (PES) of ground-state trans-azobenzene (TAB) has been explored as a function of symmetric dual C-N twist angle of the two phenyl rings about the C-N bonds in ab initio MP2/6-31 + G* calculations. Two equivalent nonplanar minima have been located in which phenyl rings are lifted from the planar structure by 17.5degrees, The two minima are connected through a planar, transition-state structure with a potential barrier of 0.35 kcal/mol. The barrier is small enough to be overcome at room temperature although the minima hold a total of eight vibrational states below the barrier. (C) 2002 Elsevier Science B.V. All rights reserved.