Davidson and co-workers (Phys. Rev. A 1991, 44, 7071; 1993, 47, 3649) have estimated the nonrelativistic correlation energies and relativistic corrections to ionization potentials for atomic ions with up to 18 electrons. However, due to the lack of theoretical values for the high-Z limits and lack of more accurate compilations of experimental ionization potentials, the analysis for 11-18 electrons required further investigation. In this work, we have accurately determined the exact high-Z limit employing degenerate second order perturbation theory for the correlation contribution to the energies of atomic ions with 3-18 electrons. We have also incorporated the experimental compilation of the electron affinity data of Hotop and Lineberger for the low-Z limit. This high-Z limit is compared with the results of the LYP correlation energy functional. The LYP correlation functional is also compared with the correlation energy of electrons in an external harmonic potential of infinite force constant.