The new method, called effective group potential, which generalizes the concept of effective core potential by including in a potential the effects of many atoms ligand, is used to calculate transition energies. Three molecules containing a metal atom (Cu, Cr, Ni) surrounded by NH3 ligands have been studied. It is shown that by replacing the NH3 group by an effective group potential, the characteristic transitions energies are very well reproduced at various ab initio levels. The transitions studied are transitions between open shells on the metallic center but also ligand to metal transitions where, though unexpected, quite reliable results were also obtained. This method is thus very well suited for the study of the spectrum of a large molecule under the condition that a separation between an active part plus some spectator groups can be made, a condition satisfied in most organometallic molecules as well as in biological molecules.