This chapter indicates that the important correlation corrections is obtained from a perturbation theory study of both the Hartree-Fock approximation and the exact wave function. Hartree (1957) was interested in the Z-dependence of radial functions as a means of obtaining good initial estimates. The MCHF atomic structure package incorporates an extension which allows a limited degree of non-orthogonality which sometimes is used as an advantage. Generally, the orbital label consists of two characters like a set of N quantum number and optionally a third character that represents a set indicator. Whenever an orthonormal radial basis is present, the energy must be stationary with respect to a rotation of the basis. Koopmans’ theorem was used to select the basis that minimized the energy of the ion. The chapter concludes that in most situations the dominant component of the wave function has spectroscopic orbitals similar to Hartree-Fock orbitals.