The effect of vibrational Zero Point Energy (ZPE) on the torsional barriers of trans-stilbene is studied in the adiabatic approximation. The two torsional modes corresponding to phenyl rotation are explicitly separated, and the remaining modes are treated as normal coordinates. ZPE reduces the adiabatic barrier along the in-phase torsion from 198 to 13 cm(-1). A one-dimensional adiabatic potential for the anti-phase torsion, including the ZPE of the in-phase torsion, reduces the adiabatic barrier from 260 to 5 8 cm(-1). Comparison with recent electronic structure benchmark calculations suggests that vibrational corrections play a significant role in trans-stilbene's experimentally observed planar structure. (c) 2007 Elsevier B.V. All rights reserved.