I use Density Functional Theory (DFT) to calculate the electronic properties of doped molecules/materials. By doping, I mean both substitutional doping and interstitial doping. At present, I optimize the geometry before and after doping. I select DFT functionals based on the previous literature. Geometry optimization using DFT basically gives the ground state structure which is the most stable structure. But some papers report formation energy/formation enthalpy of the molecules before and after doping to show that the particular structure is stable after doping.
Why calculation of the formation energy is important after doping? Why geometry optimization itself is not enough to show that the material is stable after doping?