Secondary interactions and bond critical points in ionic crystals

The sufficiency of the condition of the existence of a (3,-1) (bond) critical point of charge density between atoms in a crystal for a bonding interaction between these particular atoms has been studied for the example of the ionic LiF and NaF crystals. Results of calculations of ionic lattice energies using the experimental (X-ray diffraction) pseudoatom static charge densities, and the crystal chemistry consideration of these rock-salt crystals, favors the insufficiency of this condition in the case of secondary interatomic interactions. In general, this situation in which a bond critical point arises between two ions displaying a secondary interaction, while a chemical bond itself is absent, seems to be quite possible for crystals formed from ions with noticeably different sizes.