The effective group potentials (EGP) approach has been successfully used for the computation of the ground and excited states energies of the mixed valence compound [Fe-2(OH)(3)(NH3)(6)](2+). It is the first time that for a system as big as the complex presented above the ground and excited states are computed with their own orbitals and studied in such a detailed way. First of all, the NH3 EGP was validated by comparing calculations where NH3 was treated explicitly at different levels of calculations. Once the validation was obtained, the complete spectrum of the compound under interest was calculated and compared with results obtained in a previous work by Barone et al. and the spin Hamiltonian of widespread use. Some deviations from these predictive approaches were observed. This allowed us to emphasize the importance of the dynamic correlation which is not included explicitly in the spin Hamiltonian. Then, the influence of vibration has been studied by computing the potential energy curves obtained when moving the (OH)(3) plane. This study shows that our calculations lead to a delocalized compound (class III) as expected according to former experimental data.