[Pw_forum] Unit-cell and Super-cell bandgap difference

[dario rocca]

Dear Vipul,
the difference (0.4 eV) in energy between the cell and the supercell is
sizeable.You should try to understand this problem first of all.
To compare you don't need to relax. If you relax two equivalent systems
(cell and 2x2x2 supercell) you should obtain consistent results,
namely that the supercell has double the lattice parameter of the cell
Best
Dario

On Mon, Nov 28, 2016 at 6:08 AM, Vipul Shivaji Ghemud <
vipul at physics.unipune.ac.in> wrote:

> Thank you Dario, I'll take a run with same k-mesh. The energy of supercell
> is 0.4eV less than the 8times that of unit cell. I have just taken scf
> (without relax)
>
> > Dear Vipul,
> > There could be two main explanations for your results:
> > -A problem with the k-points you use  to compute the bandstrucure: the
> > > grid
> > for the cell and supercell must be consistent
> > -An error in the supercell. Try comparing total energies: does the
> > supercell have 8 times the total energy of the cell?
> > Best,
> > Dario Rocca
>
> > On Fri, Nov 25, 2016 at 7:47 AM, Vipul Shivaji Ghemud <
> > vipul at physics.unipune.ac.in> wrote:
>
> > I have been using 8*8*8 for unit cell and 4*4*4 for supercell as my
> > supercell is 2*2*2 unit cell.
> >
> > > Dear Vipul,
> > > how many k-points did you use for the cell and for the supercell
> > > Best
> > > Dario
> >
> > > On Thu, Nov 24, 2016 at 6:23 AM, Vipul Shivaji Ghemud <
> > > vipul at physics.unipune.ac.in> wrote:
> >
> > > Hi all,
> > > I am working on a system of 9atoms in a unit-cell having bandgap of
> > 3.5eV;
> > > but I am considering the super-cell(4 unit-cells) then the bandgap is
> > > reduced by ~0.45eV for HSE06. The bandgaps are similar with GGA. Is it
> > due
> > > to the exchnage-correlation contribution of the increased number of
> > > electrons in the system, as the code QE considers unit-cell and
> > super-cell
> > > both a a single system? It's a bulk cubic system. I am facing similar
> > > problem with other systems also.
>
>
> --
> Vipul S. Ghemud
> Ph.D. student.
> Dept of Physics,
> SPPU, Ganeshkhind,
> Pune- 411007.
>
>
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