1. Concerning more detailed information, the reader should refer to various textbooks on quantum chemistry, e.g. Daudel R., Lefebvre R., Moser C.: Quantum Chemistry. Interscience, New York 1959.
2. Turner D. W., Baker C., Baker A. D., Brundle C. R.: Molecular Photoelectron Spectroscopy. Wiley-Interscience, London 1970.
3. Löwdin P.-O.: Phys. Rev. 97, 1474 (1955).
4. McWeeny R.: Rev. Mod. Phys. 32, 335 (1960).
5. McWeeny R., Sutcliffe B. T.: Methods of Molecular Quantum Mechanics. Academic Press, London and New York 1969.