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

[poonamsharma]

Dear Experts,
Thank you so much for your quick response and useful information.
I am going to try these suggestions.
I'll update here if I'll get the correct results.
Thanks once again.

Thanks and regards,
Poonam S.



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



On Thu, Nov 11, 2021 at 1:08 PM poonamsharma <poonamkaushik40 at gmail.com>
wrote:

> 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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