Superimposed is the QTAIM bonding paths (white lines) and the bond critical points (green dots). Between the upper central hydrogens there is an artefactual bond path and critical point. As useful and appealing as it is, the structural approach is not flawless. Even after all its successes we can find cases of false positives (false bonds where there is no real bond) and false negatives (undetected bonds). Phenanthrene is one of the classical situations of the former. As shown in Fig. , the density coming from the two upper central hydrogens creates a bonding path between them, giving the appearance of a weak but real bond according to the structural definition of the bond (see the purple surface, and the green critical point). The same effect would be seen if we squeeze two noble gas atoms, where a density build-up appears between them despite being a repulsive interaction. This is a simple mathematical effect of atomic densities brought together, as shown in Fig. with two s-type atomic functions. Even without any potential energy attraction or kinetic energy interference, there will be a low-density critical point, not much different from the H-H one in phenanthrene. Some of these difficulties can be eliminated by ad-hoc impositions to the definition of the structural bond, like asking not to contract the bonds beyond their minimum energy. This condition is too restrictive, and resembles the problem of the atoms being forced in place that appeared in the energetic approach to the bond (see above).