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

[Mahmoud Payami Shabestari]

For the following rule of thumb, I had implicitly assumed rectangular 
parallelepiped structure. For non-orthogonal lattice vectors one should use 
their proper perpendicular projections, say |b2,perpend|/|b1|, etc.

Mahmoud








Dear Poonam,

Hi.

For having a uniform k-mesh in the reciprocal space, one has to respect the 
following rule of thumb:

1- For direct lattice vectors (a1, a2, a3) obtain the reciprocal lattice 
vectors (b1, b2, b3).
2- By finding the ratios |b2|/|b1| and |b3|/|b1|, you will be able to choose 
the number of k-mesh in b2 and b3 directions if you have already chosen in 
b1 direction.
HTH


Bests,
Mahmoud Payami
Physics & Accelerators Research School
NSTRI, AEOI, Tehran, Iran


Email: mpayami at aeoi.org.ir
Phone: +98 (0)21 82066504
--------------------------------------------------------




From: poonamsharma <poonamkaushik40 at gmail.com>
To: Quantum ESPRESSO users Forum <users at lists.quantum-espresso.org>
Date: Thu, 11 Nov 2021 13:08:58 +0530
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> 
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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