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

[Vipul Shivaji Ghemud]

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.