Relativistic contributions to ionization energies and bond lengths

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Abstract
The interchange theorem of double perturbation theory permits two equivalent approaches for the determination and discussion of relativistic changes of ionization energies and bond lengths. Electron density differences due to ionization and bond length variation, respectively, at the nonrelativistic level, or alternatively density differences due to relativity, have to be taken into account ; either may be used for explaining the changes in ionization energies or bond lengths. Relativistic bond length contractions and relativistic orbital contractions are related to each other. But one effect is not the cause of the other. The relation, although more complicated than originally anticipated, offers a simple explanation for relativistic bond length changes within both the energy and the force concepts of the chemical bond. Corresponding model calculations at the semiempirical as well as at the ab initio Dirac-Fock level are presented and analysed.