[QE-users] Different band gap in unitcell and in supercell

[poonamsharma]

Dear Sir,

>The band gap should not change: in a supercell you will have more electronic
>bands but the band gap must remain the same. Again, please check that you do
>all properly or share your files.

Thank you so much fr this suggestion.

 I guess I am taking wrong K points. Also I didn't perform the K
points convergence test for supercell. This may be one the reson of
this discrepancy. I'll chk this again, also I am thinking to perform
convergence test for each supercell.

Only one query I have, my a,b,c parameters for unitcell are 13.764,
3.699, 3.95. It is bulk system. For K points convergence wrt energy, I
took K points sets like 2x2x2 3x3x3... like this , and at 5x5x5 I
obtained a good convergence. Since my a,b,c are not equal so I am
taking in SCF calculation 5X10X9.

This 5X10X9 set is correct to to take although at 5X5X5 I obtained the
convergence.

Thank you so much once again.


Thanks and regards,

Poonam Sharma





-------------------------------------------------------------------------------------------------
Poonam Sharma
Research Scholar
Department of Physics
Indian Institute of Technology Bombay
Mumbai - 400076
India.




On Sun, Nov 7, 2021 at 9:17 AM poonamsharma <poonamkaushik40 at gmail.com>
wrote:

> Dear users,
> I am trying to calculate the bandgap for one system in the unit cell and
> in the supercell of 2x2x1 and 3x1x1. Although 3x1x1 gives the same bandgap
> as I got from the unit cell (1.80 eV) but 2x2x1 is giving 1.75 eV. I kept
> the input files settings same in supercells as in-unit cell including K-
> POINTS. I observed this bandgap by plotting the dos. I think the bandgap
> should remain the same in the supercell.
> Is this 1.75 eV is acceptable.
> Also in order to match this bandgap with the experimental value, I applied
> the U parameter. In unit-cell with different U values, I got a variation in
> band gap that I am expecting but in supercell, bandgap remains the same, no
> variation observed.
> I'll be very thankful for any suggestions.
>
> Thanks and regards,
> Poonam Sharma
>
>
>
>
> -------------------------------------------------------------------------------------------------
> Poonam Sharma
> Research Scholar
> Department of Physics
> Indian Institute of Technology Bombay
> Mumbai - 400076
> India.
>
>
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