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The computer-assisted C-matrix, L\"owdin-alpha-function, single-center expansion method in spherical harmonics has been applied to the three-center nuclear-attraction integral (potential due to the product of separated Slater-type orbitals). Exact formulas are produced for 13 terms of an infinite series that permits evaluation to ten decimal digits of an example using $1s$ orbitals.
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Herbert Jones (1984) studied this question.
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