Where Do the Forces in Molecules Come from? A Density Functional Study of N2 and HCl

Though chemical forces in molecules have been of considerable interest for decades, the question of the origin of such forces concerning different spatial regions in a molecule is still open to discussion. In this paper we investigate chemical forces by a density functional study of two molecules, N2 and HCl. On the one hand, we find that the quantitative details of slight polarizations of the electronic densities in the atomic core regions yield major contributions to the forces, and this is responsible for the difficulties in calculating reliable forces from approximate electron densities. On the other hand, the qualitative topology of the electron density in the valence region is not at all unimportant for electrostatic forces, and this is responsible for the possibility of a qualitative chemical interpretation of electrostatic binding.