A strategy to quantify the location of the FERMO was developed, leading to the construction of the MOLPROJ software, based on the use of projection operators to build the molecular orbitals (MOs) by linear combination of atomic orbitals (AOs; the LCAO approach). In this same study , it was possible to determine the reaction site of a series of amines and describe their acid-base behavior. Thus, the location of the FERMO would indicate the orbital in which the reaction occurs and, consequently, would point to the most favorable location for protonation.