1. The early stage of the π-electron theory has been summed up in B. Pullman and A. Pullman: Quantum Biochemistry. New York:Wiley-Interscience 1963; results of the various refinements in the π-electron approximations may be found for instance in B. Pullman: J. Chem. Phys. 43, 233 (1965).
2. Hoffmann, R.: J. Chem. Phys. 39, 1397 (1963).
3. Pople, J. A., Santry, D. P., Segal, G. A.: J. Chem. Phys. 43, 129 (1965).
4. -, Segal, G. A.: J. Chem. Phys. 43, 136 (1965); 44, 3289 (1966).
5. Pullman, A.: Int. J. Quantum Chem. 2S, 187 (1968).