Bonding indicators from electron pair density functionals

The bonding analysis of a chemical system is usually based on some descriptors. Distinct approaches are used to generate the bonding descriptors, whereby the usefulness of a particular approach is emphasized by the desire to yield a description consistent with the examined effects. Thus, whereas the bond path from the electron density gradient field yields the connectivity of the atomic fragments, the orbital picture can easily rationalize the rotational rigidity of a double bond. On the other hand none of the former is able to describe the volume demand of the bonds, which can be accessed by descriptors originating from approaches using space partitioning. As a conceivable way to describe the bonding situation a class of functionals based on the electron pair density integrals in both, direct and momentum space, is proposed. The localizability indicators defined by those functionals are examined on several molecules.