Hylleraas correlation in two-electron spectra

Electron correlation is a method of obtaining a more accurate solution of the two-electron Schrodinger equation. In the Hylleraas type correlation method, the relative electronic coordinates are introduced into the wave function, and the variation principle is used to determine some adjustable parameters. Lennard-Jones and Pople and Brickstock and Pople have developed electron correlation in twoelectron systems in a more general way for both the ground states and excited states. The Hylleraas correlation is actually a special case of the general development. In this work, a Hylleraas-type correlation function is used to modify multiplet wave functions constructed from 2 hydrogenic orbitals of the (n, n - 1) configuration outside of closed shells. The effect upon the multiplet energy spacings of a two parameter correlation function is important to determine the feasibility of similar calculations involving a more sophisticated but more accurate basis set of functions, such as the Hartree-Fock orbitals.