1. See for instance: Ladik, J.J. (1988) Quantum Theory of Polymers as Solids, Plenum Press,New York - London, p168.
2. Löwdin, P.-O. (1956) Adv. in Physics
5, 1.
3. Del Re, G., Ladik, J.J., and Biczo, G. (1967) Phys. Rev. 155, 997.
4. André, J.-M, Gouverneur, L. and Leroy, G. (1967) Int. J. Quant. Chem. 1, 427, 451.
5. Löwdin, P-O. (1951) J. Chem. Phys. 18, 365.