Charge and momentum densities of cubic tetracyanoethylene and its insertion compounds

Charge and momentum electron densities provide complementary views of cohes ive forces in solids. This is particularly true for molecular crystals. The examples of cubic tetracyanoethylene (1,1,2,2-ethenetetracarbonitrile) and its alkali-metal insertion compounds are analyzed from a theoretical point of view. Besides the usual deformation density maps and anisotropy of Comp ton profiles, it is shown that interaction charge density and interaction C ompton profiles can be defined and reveal the subtleties of the intermolecu lar interactions. It is shown that owing to the large cavities in the cryst al, alkali-metal atoms can be inserted, leading to a strong charge transfer to the molecules and to a metallic character; the mechanism of insertion i s revealed well by the combination of charge and momentum density studies. The combination of the two techniques of X-ray diffraction and Compton scat tering should be of great help in the study of rather weak interactions pre sent in molecular solids.