Impact of the Heitler--London hydrogen molecule paper on chemistry

This year marks the fiftieth anniversary of the justly celebrated paper by Heitler and London on the quantum mechanical treatment of the hydrogen molecule. A brief survey is given of some of the more important papers built on the Heitler-London classic. In addition, the question of the physical interpretation of approximate wave functions is discussed and some of the pitfalls exemplified. Several different analyses of the bonding are reviewed, and it is concluded that the most satisfactory physical interpretation is that based on the Hellmann-Feynman theorem: the true wave function is the function which minimizes the sum of the potential and kinetic energies. This causes some excess charge in the central region so as to minimize the kinetic energy at least cost in potential energy. This excess electronic charge holds the nuclei together, much as was postulated by G. N. Lewis