We can draw a line of maximum electronic density between the atoms (white lines in Fig. ), and in the middle we can mark the point of minimal density in this coordinate (but maximum density perpendicularly, see green dots in Fig. ). According to QTAIM, these lines and points are the most faithful characteristic of the chemical bond. Moreover, cutting through these critical points we can distinguish one atom from the other in the molecule, providing a compelling (but controversial and hardly universally accepted) answer to the question of what is an atom in a molecule. Because of this, QTAIM is the pet model of the structural approach to the bond (there are many other techniques that also rely on mathematical acrobatics based on electronic densities such as ELF, NCI, IQA, LED, DORI, etc., each one of them with its own advantages). in the volume). At 60% (red spheres at the nuclei positions) it is possible to see that most of the electrons are into the carbon and hydrogen cores. At 70% (orange surfaces) we start to see the atomic pairwise interactions. At 80% and 90% (yellow and blue) the structure of the bonds as a pairwise interatomic interaction is clear. At 99% (purple) the pairwise bonding starts to be undefined.