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

[Iurii TIMROV]

Dear Poonam Sharma,


Regarding the convergence with respect to the k points mesh: I would suggest to have a look at the Quantum ESPRESSO input generator https://www.materialscloud.org/work/tools/qeinputgenerator

You just upload your structure and choose between "normal", "fine", and "very fine" k points mesh. Even if the lattice parameters a, b, and c are all different, this tool will automatically find out which k points mesh to use. Then you can vary these k points meshes a bit and see how the results change, and eventually see when your results converge with a desired accuracy/precision.


HTH


Iurii


--
Dr. Iurii TIMROV
Senior Research Scientist
Theory and Simulation of Materials (THEOS)
Swiss Federal Institute of Technology Lausanne (EPFL)
CH-1015 Lausanne, Switzerland
+41 21 69 34 881
http://people.epfl.ch/265334
________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of poonamsharma <poonamkaushik40 at gmail.com>
Sent: Thursday, November 11, 2021 8:38:58 AM
To: Quantum ESPRESSO users Forum
Subject: Re: [QE-users] Different band gap in unitcell and in supercell


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<mailto: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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