[QE-users] gradients with charge compensation

Paolo Giannozzi p.giannozzi at gmail.com
Fri May 4 10:56:14 CEST 2018

The introduction of a compensating background is needed because the energy
of a charged supercell is infinite. In the case of a charged defect, the
supercell of charged defects + compensating background will converge to the
isolated defect in the limit of large supercell, but the convergence will
be slow. Various techniques (Makov-Payne, Martyna-Tuckerman, ...) can be
used to obtain better results for reasonably-sized cells. Note that
"tot_charge=+1" means "one electron missing from the system".

I have no reason to think that forces in charged supercells are wrong (not
just approximated: wrong). Your results look strange, but it is hard to
draw any conclusion from a comparison of two very different approaches and
XC functionals. If you can provide your data, maybe we can figure out what
happens and why.


On Wed, Apr 25, 2018 at 1:03 PM, Aleksandra Oranskaia <
aleksandra.oranskaia at kaust.edu.sa> wrote:

> Hi dear users and developers of QE,
> If one is interested in studying charged state defects -- is it correct to
> run supercell optimizations with a compensating jellium background ?
> Let’s say, if one is interested in +1 point defect state -- is it correct
> to add tot_charge = +1 ?
> I quickly checked if the gradients for molecular cation optimization are
> reasonable with such an approach. For this I took small cation with known
> structure and:
> - gaussian09-d.01 optimization with 6-11G**/B3LYP, +1 charge, 1 spin
> multiplicity gave perfectly matching structure with the real one
> - QE-6.2.1 optimization with vdw-df2/USPP and tot_charge = +1 destroyed
> the cation, it dissociated; without “jellium” the optimized structure is
> very close to the real one.
> I would greatly appreciate your advice on how to obtain realistic
> structures of the supercells with charged defects,
> Thanks in advance,
> Alex.
> ___
> Aleksandra Oranskaia (M.Sc.)
> ChemS PhD student, KAUST
> Phone: +966 50 1335254
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Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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