1. Cook, D. B.: Structures and Approximations for Electrons in Molecules. Chichester: Ellis Horwood 1977
2. Löwdin, P.-O., see for example Research Report No. 40 of Quantum Theory Project, University of Florida (1963)
3. Pople, J. A., Segal, G. A.: J. Chem. Phys. 43, 5129 (1965)
4. Cook, D. B.: Theoret. Chim. Acta 40, 297 (1975)
5. Cook, D. B., Hollis, P. C., McWeeny, R.: Molec. Phys. 13, 573 (1967)