Electron density and the chemical bond. A reappraisal of Berlin's theorem

An increase in the electron density in the binding region is often assumed to be essential to the stability of the nuclei in chemical bonds. This is not justified by the theorem on which the assumption is based, which demands only that there be electron density in the binding region. In its simplest form the theorem places few constraints on the deformation density, which differs markedly in character for different molecules. To understand the various ways of achieving stability of the nuclei it is necessary to study both the size and the location of features in the deformation density. The effect on binding is large only for features close to the nuclei.