1. G. Hagg, Z. Phys. Chem. B6 (1929) 221.
2. I. N. Levine, “Quantum Chemistry”, 4th edn (Prentice Hall, 1991) p. 282.
3. Idem, ibid. p. 285.
4. E. Wigner and F. Seitz, Phys. Rev. 43 (1933) 804.
5. L. Pauling, “The nature of the chemical bond”, 2nd edn (Cornell University Press, New York, 1940).