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<DIV>Let¡¯s consider positive point charges q in one dimensional lattice, that
is, at x=a*j, j=-infinity,...,-2,-1,0,1,2,...,+infinity. In order to calculate
the electrostatic energy due to Coulomb interaction between the charge at x=0
and all the other charges, one should calculate the summation: 1/a * sum_j
q^2/j. However, the summation is not converged and is positive infinite. In
summary, electrostatic interaction energy for charged system is infinite and one
needs some other technique to calculate it.</DIV>
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<DIV>Yun-Peng</DIV>
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<DIV style="font-color: black"><B>From:</B> <A title=daijiayu@nudt.edu.cn
href="mailto:daijiayu@nudt.edu.cn">jiayudai</A> </DIV>
<DIV><B>Sent:</B> Saturday, January 14, 2012 10:17 PM</DIV>
<DIV><B>To:</B> <A title=pw_forum@pwscf.org
href="mailto:pw_forum@pwscf.org">pw_forum@pwscf.org</A> </DIV>
<DIV><B>Subject:</B> Re: [Pw_forum] Ewald and Coulomb</DIV></DIV></DIV>
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<P>Dear Yun-Peng,<BR><BR>Thanks for your explanation. In fact, what i mean is
that how to treat the ion-ion interactions with some charges. For example,
sometimes we want to take out one or more electrons out of the system, thus the
tot_charge in the system is not zero. In an extreme case, all electrons are
ionized and taken out, there are only positive ions in the system. In this case,
the Ewald potential should not be right but the real Coulomb potential should be
correct. Since Ewald scheme considers the screnning by the electrons. Thus, i
want to use the exact 1/r potential to represent the Ewald scheme. So, how can
we reach this goal?<BR><BR>Best
wishes.<BR><BR>Jiayu<BR><BR><BR>>>>>>>>>>>>>>>>>>>><BR>what
do you mean by "true Coulomb potential"? Based on density functional theory,
adding an uniform potential to the system make no difference. In fact, the
ion-ion interaction energy is an infinite value because of 1/r type of Coulomb
potential. However, if an uniform charge density which makes total charge zero,
hence uniform Coulomb potential is added to the system, the electrostatic energy
as well as potential is finite, at the same time, physics keep
unchanged.<BR>best wishes,Yun-Peng</P>
<P>Date: Fri, 13 Jan 2012 21:49:59 +0800<BR>From: <A
href="mailto:daijiayu@nudt">daijiayu@nudt</A>.edu.cn<BR>To: pw_<A
href="mailto:forum@pwscf.o">forum@pwscf.o</A>rg<BR>Subject: [Pw_forum] Ewald and
Coulomb</P>
<P><BR>Dear users and developers,</P>
<P>Happy new year!</P>
<P> </P>
<P>I have a confusion about the calculations of ion-ion interactions. We know,
we usually use Ewald scheme to represent the real Coulomb potentials in a
periodic cell. Generally, it is correct for a neutral system or one electron
taken out (or into ) system. However, if the system is constructed with
partially charged ions, that is to say, there are more positive charges than
negative charges, the Ewald scheme should be not right. Although this system is
not stable, but there should be some properties deserved to study.</P>
<P>So, how can we calculate the true Coulomb potentials in DFT? That is to say,
we do not use Ewald, but only use th 1/r type. I know it can be realized in
classical calculations, but i did not find the path to get it in QE.</P>
<P> </P>
<P>Thanks a lot.</P>
<P> </P>
<P>Jiayu<BR><SPAN style="DISPLAY: none" _fck_bookmark="1"> </SPAN></P>
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