1. E. Schrödinger, Ann. d. Physik, 81, 109 (1926).
2. P. A. M. Dirac, “The Principles of Quantum Mechanics”, fourth ed., Oxford University Press, Oxford, UK, 1958.
3. R. F. W. Bader and P. F. Zhou, Chem. Phys. Letters, 191, 54 (1992).
4. M. Phillips, Encyclopedia of Physics, vol. IV, edited by S. F. Fluegge, Springer, Berlin, 1962, p.25
5. H. Fuess, The Application of Charge Density Research to Chemistry and Drug Design, edited by G. A. Jeffrey and J. F. Pinella, Plenum Press, London, 1991, p. 225.