The binding energies for the reaction M+ + CH2=CHCH3 -> M+[CH2=CHCH3], for M = Ag, An, are predicted using both second order perturbation theory (MP2) and coupled cluster theory (CCSD(T)). Systematically improving the quality of the atomic basis set to the complete basis set limit produces a monotonic improvement in the predicted binding energy for M = Ag relative to the experimentally determined value; the final predicted binding energy is within experimental error. Conversely, the same systematic improvement in the atomic basis set results in successively worse agreement at the MP2 level of theory. (c) 2005 Elsevier B.V. All rights reserved.