The topological and energetic properties of the electron density distribution ρ(r) of the isolated pairwise HF interaction have been theoretically calculated at several geometries (0.8<d<2.5 Å) and represented against the corresponding internuclear distances. From long to short geometries, the results presented here lead to three characteristic regions, which correspond to three different interaction states. While the extreme regions are associated to pure closed-shell (CS) and shared-shell (SS) interactions, the middle one has been related to the redistribution of ρ(r) between those electronic states. The analysis carried out with this system has permitted to associate the transit region between pure CS and SS interactions to internuclear geometries involved in the building of the H–F bonding molecular orbital. A comparative analysis between the formation of this orbital and the behavior of some characteristic ρ(r) properties has indicated their intrinsic correspondence, leading to the definition of a bond degree parameter [BD=HCPCP;HCP and ρCP being the total electron energy density and the electron density value at the HF (3,−1) critical point]. Along with the isolated pairwise HF interaction, 79 X–HF–Y (neutral, positively and negatively charged) complexes have been also theoretically considered and analyzed in terms of relevant topological and energetic properties of ρ(r) found at their HF critical points. In particular, the interaction energies of X–HF–Ypure CS interactions have been estimated by using the bond degree parameter. On the other hand, the [FHF] proton transfer geometry has been related to the local maximum of the electron kinetic energy density (GCP)max.

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