[Pw_forum] FW: about Fermi Energy shift in periodic (q=0) perturbations

[Ezad Shojaee]

Dear Mr. Gironcoli
Thanx for your answer, but sorry I'm not sure i follow you, could you open it more?!
you mean in the case of periodic perturbation(like the constant one) we have a change in the # of electrons? what exactly happens in this case, so we have to shift the Fermi energy(in order to have neutrality condition)? What happens if we do not shift it(As you see i want you to answer the question you made!)?
Thanx again
 
Ezad Shojaee - Tehran University
 
> At any q/=0 the perturbation is such that the number of electrons does > not change.> Even in the limit of q->0 the macroscopic term in the density response > is zero (because in a metal> the dielectic constant is infinite).> Now consider the case of a periodic (q=0) perturbation, the simplest > one, a constant, threated with or w/o the FE shift... which of the two > treatments corresponds to the result you would obtain in the liimit q->0 ?
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