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

[Iurii TIMROV]

Dear Poonam Sharma,


> 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.


This is strange. Are you sure that you do things properly? Can you share your input and output files for a primitive cell and a supercell? Which version of QE do you use? Please check posting guidelines:  https://www.quantum-espresso.org/forum


> 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.


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.


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: Sunday, November 7, 2021 4:47:57 AM
To: Quantum ESPRESSO users Forum
Subject: [QE-users] Different band gap in unitcell and in supercell

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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