1. L.I. Schiff: Quantum Mechanics (McGraw-Hill Book Company, New York, 1955), Chapter 12
2. H.A. Bethe, E.E. Salpeter: Quantum Mechanics of One-and Two-Electron Atoms (Springer, Berlin, Heidelberg 1957)
3. J.C. Slater: Quantum Theory of Atomic Structure, Vol. II (McGraw-Hill Book Company, New York 1960), Chapter 24
4. M. Blume, R.E. Watson: Proc. Roy. Soc. A270, 127 (1962); A271, 565 (1963)
5. P. Hohenberg, W. Kohn: Phys. Rev. 136, B864 (1964)