The equilibrium geometry. ring interconversion barrier, and pathway for cyclohexene have been studied using MP2 and DFT(B3LYP) methods with different basis sets. It is demonstrated that the energy profile for ring interconversion is extremely top-flattened. The transformation from one twist-boat conformation to another via boat occurs without a change in energy. The size of this plateau on the potential energy surface is about +/-40degrees. Therefore, the cyclohexene ring interconversion should be described as a three-stage process including almost free transformation of one twist-boat conformation to another. (C) 2002 Elsevier Science B.V. All rights reserved.