nbnd in PWscf calculations

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8813204602
Posts: 18
Joined: Tue Jun 15, 2021 5:53 pm

nbnd in PWscf calculations

Post by 8813204602 » Mon Oct 04, 2021 10:49 pm

Dear developers
I want to calculate the band structure for a semiconductor material with a 2eV gap. When perform nscf calculation to make a Yambo input file,using nbnd=20, the output is correct and show a semiconductor material with 15 occupied bands. But when increase nbnd, for example nbnd=400, the output shows a half-metall material,that means 16 and 17 bands are partially occupied. Why does this happen by changing the value of nbnd?

Best regards,
Mitra Helmi
Mitra Helmi,Ph.D student
Ferdowsi university of Mashhad
Mashhad,Iran

andrea.ferretti
Posts: 195
Joined: Fri Jan 31, 2014 11:13 am

Re: nbnd in PWscf calculations

Post by andrea.ferretti » Tue Oct 05, 2021 7:04 am

Dear Mitra,

as you mention, such a behaviour is not expected (the only reason I can come up with is e.g. a problem in the nscf diagonalization with 400 bands).
In order to go any further, it would be useful if you could send scf+nscf input and output files showing the problem such that we can try to reproduce it.

take care
Andrea
Andrea Ferretti, PhD
CNR-NANO-S3 and MaX Centre
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322; Skype: andrea_ferretti
URL: http://www.nano.cnr.it

8813204602
Posts: 18
Joined: Tue Jun 15, 2021 5:53 pm

Re: nbnd in PWscf calculations

Post by 8813204602 » Tue Oct 05, 2021 6:46 pm

Dear Andrea

scf and nscf input and output files are attached.

There is no problem up to nbnd=75, but from nbnd=80,partial occupation of 16 and 17 bands is seen in the nscf output and value of Fermi level also changes.

Thanks a lot
Mitra
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Mitra Helmi,Ph.D student
Ferdowsi university of Mashhad
Mashhad,Iran

andrea.ferretti
Posts: 195
Joined: Fri Jan 31, 2014 11:13 am

Re: nbnd in PWscf calculations

Post by andrea.ferretti » Thu Oct 07, 2021 6:33 pm

Dear Mitra,

I had a look at your inputs in order to reproduce the problem. Here are some comments
  • since you are using norm-conserving pseudo potentials, ecutrho should be exactly 4 times ecutwfc (or simply
    unset, leading to the default that is 4*), while you are setting 8 (no reason to do that, but calculations are slower).
  • force_symmorphic=.true. is only needed in the last pw.x calculation before p2y (say, the nscf); no need to set it
    at the scf level.
  • when using smearing="mp" I get spurious (and negative) occupations for bands 16,17 already at the scf level;
  • using smearing="gaussian" solves the problem in my case
  • using smearing for yambo calculations may be somehow problematic (eg the smearing shape in yambo is only Fermi-Diract, etc);
    Ideally you want to use a small value in the scf calculation in order to be able to use starting_magnetization and get to the magnetic solution you are looking for;
    Then I would run the nscf calculation without any smearing (after all, you have a semiconductor)
  • all tests were done using qe-6.7, though this is probably not causing any problem
take care
Andrea
Andrea Ferretti, PhD
CNR-NANO-S3 and MaX Centre
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322; Skype: andrea_ferretti
URL: http://www.nano.cnr.it

8813204602
Posts: 18
Joined: Tue Jun 15, 2021 5:53 pm

Re: nbnd in PWscf calculations

Post by 8813204602 » Fri Oct 08, 2021 10:05 am

Dear Andrea

Thank you very much for your useful guidances.

Best regards,
Mitra
Mitra Helmi,Ph.D student
Ferdowsi university of Mashhad
Mashhad,Iran

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