A second possibility is to disregard the cases where the density between the atoms reaches negligible values. Filtering out the cases of too low density makes a lot of sense, but degrades the significance of this definition. Fig. shows the electronic density isosurfaces of NiCO4. Between the metal and the ligands the density is much smaller than the values between C-H and C-C bonds in phenanthrene (Fig. ). This makes sense, given that the bonds are weaker. Van der Waals interactions still have a QTAIM critical point that permits their detection, but their densities at those points is extremely low, almost negligible. This is problematic, since the structural approach is based on a significant electronic build-up, not on a mathematical effect that can be fortuitous when bringing atoms around. Electrostatic interactions also show critical bonds, but they do not show any meaningful superposition between the atomic electron densities spheres of interaction. For instance, NaF does not show any contact between the atomic densities even above the 99% isosurface (Fig. ). This number is too high to justify the chemical bond as an electron density build-up model. In purely ionic bonds we must admit that whatever is making the bond, it is not something chemically tangible, but a physical coulombic force field. We may argue that opposed to covalent bonds in electrostatic interactions the 'structure' can be a build-up in the electric field, but that goes against the structural approach defined by electronic densities. In other words, the structural definition of the chemical bond is an excellent approach, but only for covalent interactions, a subset of the whole paradigm. These would be the limits of the model.