The potential-energy hypersurface of the addition reaction OH+C2H4 was partially explored following two different approaches. First, the stationary points were located at the MP2(FULL)/6-31G(d,p) level and then the minimum energy path (MEP) was built starting from the MP2 saddle-point geometry. In order to improve the energetics along the MEP, single-point calculations were carried out at several higher levels, in particular, PMP2, MP4sdtq, PMP4sdtq, and QCIsd(t). In a different approach, the C-O bond length was assumed to provide an accurate parametrization of the reaction path in the vicinity of the transition state, The minimum energy structures at the MP4sdq/6-311+G(d,p) level for 16 points along the RC-O coordinate have been calculated, followed by a generalized normal-mode analysis at the MP2(FULL)/6-311+G(d,p) level far each point. The initial potential information from both approaches was used to calculate canonical variational transition state (CVT) association rate constants for the temperature range 200-1000 K. Our calculations at the PMP4sdtq/6-311+G(d,p)//MP4sdq/6-311+G(d,p)[MP2(FULL)/6-311 +G(d,p)] level reproduce the inverse dependence of the rate constant with temperature at T <565 K, in agreement with the experimental evidence that this reaction has a negative activation energy at room temperature. The analysis of the enthalpic and entropic contributions to the Gibbs free-energy profile has allowed us to understand those negative values of the activation energy. (C) 1997 American Institute of Physics.