The projector can be understood as the projection of a "shadow" of a selected MO in the subspace of a set of AOs , thus making it a quantitative characterization of the shape of MO in a set of AOs and, consequently, a set of atoms. Thus, it is possible to define the degree of localization ΓFERMO: it is the norm of an MO projected on the expected set of atomic orbitals, which are important and that participate in the reaction of a certain compound, as follows in Equation 4 :