Wave-Mechanical Treatment of the Li2 Molecule

A treatment of the Li2 molecule by the Heitler-London method, with Slater wave functions for the atoms, has been carried out both with and without the customary approximations. In the absence of these approximations the binding computed is only a small fraction of that observed. It is found that the inner shells of the atoms may have an important effect on the magnitude of molecular binding, and may either increase or decrease it. Since their effect is associated with electron exchange it appears that they cannot be replaced by an equivalent potential field without serious error. Triple exchanges of electrons are found to be important, while polar binding is quite negligible in Li2. A criterion for the applicability of the ``interaction operator'' is developed; the error involved in its use with Li2 is small. A variational treatment has also been applied to this molecule. It yielded a binding energy less than that observed by about 0.5 e.v. The elimination of the residual error apparently requires the introduction of the interelectronic distances as coordinates. Convenient methods for the numerical treatment of multiple electron exchanges in complicated molecules are described.