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<div style="margin-top:0; margin-bottom:0">Dear QE developers,</div>
<div style="margin-top:0; margin-bottom:0"><br>
</div>
<div style="margin-top:0; margin-bottom:0"> I would like to better understanding the details of the "exx_divergence" routine and in particular the
<br>
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<div style="margin-top:0; margin-bottom:0"> implementation of the Gygi-Baldereschi scheme.
<br>
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<div style="margin-top:0; margin-bottom:0"> Following the paper by Nguyen and de Gironcoli (Phys. Rev. B 79, 205114) I understand the first part</div>
<div style="margin-top:0; margin-bottom:0"> of the routine where the reciprocal-space part of the Ewald sum for a single point charge in the</div>
<div style="margin-top:0; margin-bottom:0"> supercell is computed. As far as I understand this first part is the implementation of Eq. (17) in their paper.</div>
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</div>
<div style="margin-top:0; margin-bottom:0"> What I do not understand is the last part of the routine (I paste it below for clarity) which as far as I understand</div>
<div style="margin-top:0; margin-bottom:0"> is not discussed in the paper (or at least it is not obvious to me).
<br>
</div>
<div style="margin-top:0; margin-bottom:0"> Any hint and/or reference to help me understand it would be greately appreciated!</div>
<div style="margin-top:0; margin-bottom:0"><br>
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<div style="margin-top:0; margin-bottom:0">Thank you for your help and best regards,</div>
<div style="margin-top:0; margin-bottom:0"><br>
</div>
<div style="margin-top:0; margin-bottom:0">Nicola Colonna<br>
</div>
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<div>........<br>
!<br>
alpha = alpha / tpiba2<br>
!<br>
nqq = 100000<br>
dq = 5.0d0 / SQRT(alpha) / nqq<br>
aa = 0._dp<br>
!<br>
DO iq = 0, nqq<br>
q_ = dq * (iq+0.5d0)<br>
qq = q_ * q_<br>
IF ( erfc_scrlen > 0 ) THEN<br>
aa = aa -EXP( -alpha * qq) * EXP(-qq/4.d0/erfc_scrlen**2)*dq<br>
ELSEIF ( erf_scrlen > 0 ) THEN<br>
aa = 0._dp<br>
ELSE<br>
aa = aa - EXP( -alpha * qq) * yukawa / (qq + yukawa)*dq<br>
ENDIF<br>
ENDDO<br>
!<br>
aa = aa * 8.d0/fpi<br>
aa = aa + 1._dp/SQRT(alpha*0.25d0*fpi)<br>
IF ( erf_scrlen > 0) aa = 1._dp/SQRT((alpha+1._dp/4.d0/erf_scrlen**2)*0.25d0*fpi)<br>
div = div - e2*omega * aa<br>
!<br>
exx_divergence = div * nqs<br>
!<br>
CALL stop_clock( 'exx_div' )<br>
!<br>
RETURN</div>
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<div><font face="Helvetica"><span dir="ltr" id="divtagdefaultwrapper" style="font-size:12pt"><span style="font-size:13px"><span style="font-size:13px"><span style="font-size:13px">-----------------------------------------------------</span></span></span></span></font></div>
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<div><font face="Helvetica"><span dir="ltr" id="divtagdefaultwrapper" style="font-size:12pt"><span style="font-size:13px"><span style="font-size:13px"><span style="font-size:13px"><font color="black"><span style="text-transform:none"><font color="black"><font color="black"><span style="font-weight:normal">PAUL
SCHERRER INSTITUT</span></font></font></span></font></span></span></span></span></font></div>
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<font face="Helvetica"><span dir="ltr" id="divtagdefaultwrapper" style="font-size:12pt"><span style="font-size:13px"><span style="font-size:13px"><span style="font-size:13px">Nicola S. Colonna</span></span></span></span><br>
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<div style="text-indent:0"><font face="Helvetica">WHGA/150<br>
CH-5232 Villigen-PSI</font></div>
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