The work of Heitler and London has shown that the first order perturbation energies of two atoms taken as a function of the separation between the nuclei gives a fairly good description of the behaviour of the atoms with regard to possible molecular formation. For example, the dissociation energy of the hydrogen molecule calculated in this way is of the same order of magnitude as the accepted observed value, being in fact about 25% too small. The theory has been generalised by Heitler, and also by London and others, to the case where each atom contains many electrons, but detailed calculations do not appear to have been made for systems more complex than that of two hydrogen atoms.