1. P.A.M. DIRAC, Proc. Roy. Soc. (London) 117, 610 (1928); “The Principles of Quantum Mechanics” revised 4th edition (Clarendon Press, Oxford, 1967), ch. 11.
2. P.-O. LOWDIN, J. Mol. Spectry. 14, 131 (1964).
3. See also R.G. WOOLLEY, Mol. Phys. 30, 649 (1975).
4. H.A. BETHE and E.E. SALPETER, “Quantum Mechanics of One-and Two-Electron Atoms” ( Springer Verlag, Berlin, 1957 ).
5. G. BREIT, Phys. Rev. 34, 553 (1929); 39, 616 (1932).