Abstract: We Performed ab initio calculations on two typical S_N2 reactions (1), F_a^- +CH₃F_b→F_aCH₃+F_b^-, and (2), Cl_a^-+CH₃Cl_b→Cl_aCH₃ + Cl_b^-, for their Minimum Energy ReactionPathways (MERP).It is found that for predicting the correct MERPs and reliable energetic results of these two reactions, the diffuse functions must be included in the selected basis set, and so corrected the errors in previously calculated activation energies for these reactions, especially for reaction (1).

Key words: S_N2 reaction, Ab initio, Diffuse function