Formation of the chemical bond and orbital contraction

Various criteria for orbital contraction, most of them independent of the variational principle and based rather on formal properties of the exact wave function, all indicate that the formation of the chemical bond in ${\mathrm{H}}_2^{+}$ or ${\mathrm{H}}_2$ is accompanied by orbital contraction. Partially basing his argument on previous work by Pettifor, Heine recently criticized Ruedenberg's analysis of the chemical bond, especially the concept of orbital contraction. We show this to be due to interpretative problems of the linear-combination-of-atomic-orbital (LCAO) concept. The consequences for LCAO band-structure calculations are discussed.