1. H. Eyring, J. Walter and G. F. Kimball, Quantum Chemistry, Wiley, New York (1944), p. 351.
2. H. Margenau, Phys. Rev. 38, 347 (1931).
3. H. Hargenau, Rev. Mod. Phys. 11, 1 (1939).
4. B. C. Carlson and G. S. Rushbrooke, Proc. Camb. Phil. Soc. 46, 626 (1950).
5. P. M. Levy, Phys. Rev. 177, 509 (1969).