1. Boyd,D.B., Lipscomb,W.N.: J. theoret. Biol. 25, 403 (1969).
2. ? Hoffmann,R.: J. Amer. chem. Soc. 93, 1064 (1971).
3. Pople,J.A., Segal,G.A.: J. chem. Physics 44, 3289 (1966).
4. Pullman,A.: Int. J. quant. Chemistry 2S, 187 (1968).
5. Boyd,D.B.: In: The purines: Theory and experiment, Proceedings of the Jerusalem Symposium on Quantum Chemistry and Biochemistry, April 5?9, 1971, Eds. E. D. Bergmann and B. Pullman. New York: Academic Press. In press.