1. See McLean,A.D., Liu,B.: J. Chem. Phys. 58, 1066 (1973), for one such approach
2. Dewar,M.J.S.: The molecular orbital theory of organic chemistry. New York, N.Y.: McGraw Hill 1969
3. Salem,L.: Molecular orbital theory of conjugated systems. New York: W.A.Benjamin Inc. 1966.
4. Suzuki,H.: Electronic absorption spectra and geometry of organic molecules. N.Y.: Academic Press 1967
5. Mataga,N., Kubota,T.: Molecular interactions and electronic spectra. New York: M.Dekker, Inc. 1970