1. Coulson, C. A. ..Valence (Oxford), 1952.
2. Breit, G...Phys. Rev., 1929,34, 553.
3. —————.. Ibid., 1932,39, 616.
4. Courant, R. and Hilbert, D...Methods of Mathematical Physics (Interscience Publishers, Inc., N.Y.), 1953, p. 56.
5. Heitler, W. and London, F...Zeit. f. Phys., 1927,44, 455.