Using an asymmetric-Lanczos-chain algorithm for the calculation of the coupled cluster linear response functions at the CCSD and CC2 levels of approximation, we have calculated the mean excitation energies of the noble gases He, Ne and Ar, and of the hydrogen molecule H₂. Convergence with respect to the one-electron basis set was investigated in details for families of correlation consistent basis sets including both augmentation and core-valence functions. We nd that the electron correlation effects at the CCSD level change the mean excitation energies obtained at the uncorrelated Hartree-Fock level by about 1%. For the two-electron systems He and H₂, our CCSD results (for a Lanczos chain length equal to the full excitation space), I₀ = 42:28 eV (Helium) and I₀ = 19:62 eV (H₂), correspond to full conguration interaction results and are therefore the exact, non-relativistic theoretical values for the mean excitation energy of these two systems within the Bethe theory for the chosen basis set and, in the case of H₂, at the experimental equilibrium geometry.