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<P style="BACKGROUND: white"><SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana">Dear Mr. Gironcoli<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" /><o:p></o:p></SPAN></P>
<P style="BACKGROUND: white"><SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana">Thanx for your answer, but sorry I'm not sure i follow you, could you open it more?!<o:p></o:p></SPAN></P>
<P style="BACKGROUND: white"><SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana">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</SPAN><SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana">(As you see i want you to answer the question you made!)?<o:p></o:p></SPAN></P>
<P style="BACKGROUND: white"><SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana">Thanx again<o:p></o:p></SPAN></P>
<SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana; mso-fareast-font-family: 'MS Mincho'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1.0pt; mso-fareast-language: JA; mso-ansi-language: EN-US; mso-bidi-language: AR-SA"> </SPAN><BR>
<SPAN style="FONT-SIZE: 10pt; COLOR: #444444; FONT-FAMILY: Verdana; mso-fareast-font-family: 'MS Mincho'; mso-bidi-font-family: 'Times New Roman'; mso-font-kerning: 1.0pt; mso-fareast-language: JA; mso-ansi-language: EN-US; mso-bidi-language: AR-SA">Ezad Shojaee - <?xml:namespace prefix = st1 ns = "urn:schemas-microsoft-com:office:smarttags" /><st1:place w:st="on"><st1:PlaceName w:st="on">Tehran</st1:PlaceName> <st1:PlaceType w:st="on">University</st1:PlaceType></st1:place></SPAN><BR>
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<BR>> At any q/=0 the perturbation is such that the number of electrons does <BR>> not change.<BR>> Even in the limit of q->0 the macroscopic term in the density response <BR>> is zero (because in a metal<BR>> the dielectic constant is infinite).<BR>> Now consider the case of a periodic (q=0) perturbation, the simplest <BR>> one, a constant, threated with or w/o the FE shift... which of the two <BR>> treatments corresponds to the result you would obtain in the liimit q->0 ?<BR><BR>
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