Electron correlation in doubly excited S states: large-Z limit

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Abstract
The intrashell states of two-electron atoms are studied using the diagonalisation of the electron interaction r12-1. The relationship between the spatial structure of the electron density and electron correlation is established. (r12-1) is obtained as an asymptotic function of the principal quantum number n for the lower (r12-1)0=(1-2/ pi )n-2/5+(8 pi )n-3 and the higher (r12-1)n-1=0.5 ln 4n+0.6 states. It is shown that the screening approximation can be used for reliable non-relativistic estimation of lower state energies along the whole isoelectric series. It is predicted that for any positive ion there exists an n0, at which intrashell multiplets start to overlap.