ABSTRACT
This chapter explains the major contributions to the wave function which represents the zero-order wave function. It also discusses the contribution from correlation within the valence subshells and between valence and core subshells. According to Z-dependent perturbation theory, the complex must be part of the zero-order wave function that describes the dominant correlation effects in the atom. But this theory is valid for high Z and is not practical for neutral atoms or ions of low degrees of ionization. The author states that it is possible to describe pair-correlation functions within two-electron systems. Core-valence correlation is represented by configuration state functions (CSFs) obtained by orbital replacements from the zero-order wave function or multireference set of the type where there are respectively core and valence orbitals. In most cases core-valence correlation is correlation with CSFs outside the complex. It decreases faster, relative to the transition energy, than valence correlation that contains the important and constant contribution from correlation within the complex.
