Higher-order ion-size effects in pseudopotential theory

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Abstract
The ion size effects formulated by R.H. Bartram, A.M. Stoneham and P. Gash (1968) for colour centres, later extended by R.D. Zwicker (1975) are examined for the convergence of higher-order terms. When applied to the evaluation of the low-lying excited state energies of alkali and rare gas atoms, it is found that the convergence is very doubtful for heavier elements. This is shown to be due to the outermost electron shells with extended charge density. A hybrid scheme in which the outermost s and p shells of the ion core are represented by the exact evaluation of various terms is shown to give excellent results. Its potential application in solids is indicated.