Ab initio calculations were carried out to study the potential energy surface of (H3CHCH3)−. The 6–31G* basis set is supplemented by a set of diffuse p functions on both C and H (with a range of exponents for the latter). The binding energy of CH4 and CH3− to form the (H3CHCH3)− complex is about 2 kcal/mol, much smaller than for comparable ionic H-bonded systems involving O or N atoms. Nearly half of this interaction energy is due to correlation effects, computed at second and third orders of Møller-Plesset perturbation theory. Correlation is also responsible for substantial reductions in the energy barrier to proton transfer within the complex. This barrier is computed to be 13‒15 kcal/mol at the MP3 level, depending upon the exponent used for the H p functions.