A wide range of values for the exothermicity of the oxidative addition of methane to (cyclopentadienyl)rhodium carbonyl have been reported at the MP2 level of theory. These energies are recalculated using several different basis sets which reproduce previous results and provide evidence for the inadequacy of some commonly used basis sets. The calculations show that the magnitude of the exothermicity of this reaction is particularly sensitive to the metal 5s orbital. With a metal basis set which includes proper representation of the (n + I)s and -p metal orbitals, the exothermicity is also calculated at the MP3, MP4(SDQ), and QCISD levels. The best results show that the MPX perturbation series does not converge well and that the MP2 results in any basis set are only qualitative.