Using an asymmetric Lanczos chain algorithm for the calculation of the coupled cluster linear response functions at the
coupled cluster singles and doubles (CCSD) and coupled cluster singles and approximate iterative doubles (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 detail for families of correlationconsistent
basis sets including both augmentation and core-valence functions. We find 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 configuration 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.
