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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?

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    $\begingroup$ Geometry optimization only demonstrates metastability not thermodynamic stability. Experiments measure the formation heats that you describe (presumably you will want to compare to experiment). Moreover, energy differences are more accurate in DFT than absolute energies. $\endgroup$ – lemon Feb 27 '16 at 8:34
  • $\begingroup$ You asked this on Chemistry SE earlier this week. $\endgroup$ – Jon Custer Feb 27 '16 at 13:35
  • $\begingroup$ Yes. I wanted to transform the question to Physics SE. I have already deleted the question in Chemistry SE. $\endgroup$ – phenomenon Feb 27 '16 at 13:46
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It is important to evaluate the energetics of doping to ascertain several things. The segregation of dopants, namely their tendency to aggregate and form secondary phases. This can be examined by looking at the energetics of a pair of dopant atoms in a disperse and compact configuration, that is to say, examine whether the total free energy of a system is increased or decreased by the dopant elements moving towards each other. This is done by placing two dopant atoms in the DFT supercell. Secondly, one must examine whether energetics favour the location of the dopant element in substitutional or interstitial positions. Thirdly, it is often important to examine various dopant configuration in terms of valence, or charge state... The combination of these factors means that numerous calculations need to be performed on various configuration, of which many are possible to truly find the most likely dopant beaviour in a system..

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