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<div class="moz-cite-prefix">dear Mahmoud<br>
<br>
As Prasenjit wrote you, most probably the system will not remove
the electron from one of the two fragments only but will split it
among the two fragments.<br>
<br>
This is wrong and is due to the self interaction error present in
approximate DFT functionals.<br>
<br>
Actually this problem (partial charges on the fragments) may arise
even for neutral case and is a very important failure of
approximate DFT<br>
<br>
let's examine this latter case and let's take NaCl dimer as an
example...<br>
<br>
In nature, in vacuum, NaCl dissociates in neutral fragments<br>
<br>
This is because the Na Ionization Potential (I_Na, energy
required to extract one electron from Na<br>
is higher than Cl Electron Affinity (A_Cl, energy gained by adding
one electron to Cl).<br>
<br>
A property of Isolated systems (as the fragments are assumed to
be) is that the energy as a function of fractional occupation is
piece-wise linear w.r.t. the occupation of the last eigenvalue and
the linear slope in the total energy is the eigenvalue itself
(equal to -I_Na and -A_Cl for the two isolated fragments in our
case). I_Na > A_Cl<br>
<br>
As occupations are bound between 0 and 1 this fixes the GS to be
given by neutral fragments.<br>
<br>
However due to self-interaction error the energy vs occupation
function is not linear for approximate functionals which in turn
implies that the partially occupied eigenvalue of the fragments
depends on the occupation and it may and does happen that
HOMO_Na(x=1) > LUMO_Cl(1-x=0) ... that is there is an initial
gain in energy coming from transferring a fraction of electron
from Na to Cl...<br>
as the transfer progresses the HOMO_Na eigengalue goes down and
LUMO_Cl goes up until for a certain critical value of x (call it
xc) HOMO_Na(xc)=LUMO_Cl(1-xc) and the transfer stops ...<br>
but this means that Na has (1-xc) positive charge and Cl has
(1-xc) negative charge with xc not integer.<br>
<br>
Way to fix this requires the use of a better functional for which
the piece wise linearity is restored (with an accurate value for
the slope of course) or at least such that the HOMO_Na(x=1) >
LUMO_Cl(1-x=0) condition does not occurs and the average slope has
an accurate vale for -I_Na and -A_Cl....<br>
<br>
Hybrids functional can serve that purpose. <br>
DFT+U corrections can also address this issue if the U parameter
and the molecular orbital manifold of the fragments on which it is
applied is chosen properly.<br>
<br>
HTH<br>
<br>
stefano<br>
<br>
On 02/23/2013 04:36 PM, Mahmoud Payami Shabestari wrote:<br>
</div>
<blockquote cite="mid:WC20130223153628.501042@aeoi.org.ir"
type="cite">
<pre wrap="">Dear QE Developers and Users,
I have constructed a supercell containing two molecules "A" and "B" well
separated by, say, 15 Bohrs distance. Now I give a total charge +1 for the
cell. How the code decides to assign the charge +1 to molecule "A" or "B"?
Any comments is highly appreciated.
Best regards,
Mahmoud Payami
Condensed Matter Section
Physics Group, AEOI, Tehran-Iran
Email: <a class="moz-txt-link-abbreviated" href="mailto:mpayami@aeoi.org.ir">mpayami@aeoi.org.ir</a>
Phone: +98 (0) 21 82064393
Fax: +98 (0) 21 88221105
---------------------------------------------
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