1. Lewis, G. N.: Valence and the Structure of Atoms and Molecules. New York: The Chemical Catalog Co. 1923. Key historical references to the original papers of Lewis, Langmuir and others are found also p. 5 of Pauling's book, Ref. [2]
2. Pauling, L.: The Nature of the Chemical Bond. Ithaca, New York: Cornell University Press 1960
3. Hückel, E.: Z. Phys. 76, 636 (1932)
4. Ref. [2], pp. 387, 390
5. See e.g. Sinanoğlu, O., Wiberg, K. B.: Sigma MO Theory. Yale Press, N.Y./London, 1970