We report ab initio computations on the Hartree-Fock level and including effects of electron correlation for the following SN2 reactions: ACH3 + B- → A- + H3CB (A, B = H, F, Cl), FH + F- → F- + HF, H2 + H- → H- + H2, FSiH3 + F- → F- + H3SiF. The corresponding barriers and reaction energies (in vacuo) are determined with an accuracy of a few kilocalories per mole. It is found that electron correlation contributes up to ±7 kcal/mol to the corresponding ΔE and, consequently, cannot be neglected even in these reactions which are of closed shell type.