1. L. N. Bulaevskii and Yu. A. Kukharenko,Soviet Physics?JETP 33, 821 (1971).
2. L. Salem,The Molecular Orbital Theory of Conjugated Systems (Benjamin, New York, 1966).
3. W. A. Little and H. Gutfreund,Phys. Rev. B 4, 817 (1971).
4. D. A. Khirzhnits, E. G. Maksimov, and D. I. Khomskii, FIAN Preprint No. 108 (1970);
5. J. Low Temp. Phys. 10(1/2), 79 (1973).