[QE-users] changing cell size 1x1 to 3x3 does not provide same DOS & BS

[wenusaras]

On 2024-02-24 02:00, Lorenzo Paulatto wrote:
> Dear Wenusara,
> 
> the 3x3 supercell has a much larger amount of vacuum, this is a
> typical source of problems with graphene, as one electron may like to
> wonder out in the vacuum.  Sticking to 5-10Å of vacuum is enough and
> should work fine, i.e. when you multiply celldm(1) by 3, you should
> also divide celldm(3) by the same amount to keep vacuum constant.
> Also, your plane wave cutoff seems a bit low, I assume you have tested
> convergence properly.
> 
> kind regards
> On 23/02/2024 17:48, wenusaras wrote:
> 
>> hi,
>> 
>> I am new to QE.
>> I start with the simple pw scf code of graphene 1x1 unit cell (i.e.
>> nat=2) then the DOS and BS was obtained.
>> After that pw scf code was changed to 3x3 cell (i.e. nat =18) then
>> the obtained DOS and BS was not similar to 1x1 (changing the number
>> of kpoints was not make two BS (or DOS) similar).
>> 
>> Herewith I have attached the both pw scf & nscf files.
>> 
>> I am curious to know what is the reason for this?
>> Is there a way to make this both DOS and BS for both 1x1 and 3x3 to
>> similar?
>> 
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> 
> --
> Dr. Lorenzo Paulatto
> IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
> phone: +33 (0)1 442 79822 / skype: paulatz
> http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
> 23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
> 
> Links:
> ------
> [1] http://www.max-centre.eu
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing list users at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

____________________________________________________________________________________________________________

Dear Dr.Lorenzo,

Thank you very much for your explanatory suggestions.
As you pointed out, of course there was a bit convergence issue in both 
1x1 & 3x3 systems.
After correcting ecutwfc and maintaining a same amount to keep vacuum 
constant (i.e.,30A for 1x1 system and 10A for 3x3 syetem), the resulting 
DOS was reasonably same for both systems.
Feel free to comment.
Not only that, for the both cases the equal fermi energy was obtained.

Then the Band Structures were obtained for the both systems.
It would be a great if you could provide suggestions on, how to obtain 
more similar Band Structures for 1x1 & 3x3 systems?

Thank you for being helpful during my learning curve of QE computing.
-- 
Wenusara Satheekshana
University of Moratuwa
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