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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span lang=FR>Dear users,<o:p></o:p></span></p><p class=MsoNormal><span lang=FR><o:p> </o:p></span></p><p class=MsoNormal>I am getting confused over some pretty basic questions, when it comes to comparing the energy of two charged supercells.<o:p></o:p></p><p class=MsoNormal>When we want to compute the formation energy of a charged defect (let’s say a +1 charged vacancy), we use the following:<o:p></o:p></p><p class=MsoNormal>Ef= E(vac)-[E(bulk)-mu-Efermi]<o:p></o:p></p><p class=MsoNormal>We substract from the bulk term the chemical potential mu of the vacant species, and also the Fermi level, which is pretty much another kind of chemical potential for the missing electron at 0K. This way, we discard the energy difference that stems from taking out an electron from the arbitrary Fermi level.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Does that mean that I have to take this Fermi level into account when comparing the energy of two identically charged cells as well? For examples, let’s say I have the same compound in two different phases (A and B), both with the same +1 vacancy. The Fermi energies that I get from a scf calculation are significantly different. If I want to calculate the delta_E between both charged defective cells, I should not do dE=E_B(vac)-E_A(vac), but:<o:p></o:p></p><p class=MsoNormal>dE=[E_B(vac)+Efermi_B]-[E_A(vac)+Efermi_A] ?<o:p></o:p></p><p class=MsoNormal>This would discount the difference in energy that arises from taking out an electron from each system at different fermi levels. That would be more consistent with the above definition of vacancy formation energy for both A and B.<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Is it a correct way to compare both, or am I way off track?<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks in advance<o:p></o:p></p><p class=MsoNormal>Julien<o:p></o:p></p></div></body></html>