The method, called effective group potential, (EGP), which includes the effect of a group of atoms in a single potential, has been used successfully in a large variety of applications. Using the EGP method, optimal geometries, harmonic vibrational frequencies, activation energies, and transition energies can be safely calculated with various correlation methods even for rather large molecules. The usefulness of this method for the theoretical study of a selection of some rather difficult cases is presented. It is shown that, even if many atoms are removed from a fragment and, furthermore, if most of the electrons on the remaining atoms are removed as well, very good accuracy can be conserved.