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nbnd in PWscf calculations
Posted: Mon Oct 04, 2021 10:49 pm
by 8813204602
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
Re: nbnd in PWscf calculations
Posted: Tue Oct 05, 2021 7:04 am
by andrea.ferretti
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
Re: nbnd in PWscf calculations
Posted: Tue Oct 05, 2021 6:46 pm
by 8813204602
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
Re: nbnd in PWscf calculations
Posted: Thu Oct 07, 2021 6:33 pm
by andrea.ferretti
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
Re: nbnd in PWscf calculations
Posted: Fri Oct 08, 2021 10:05 am
by 8813204602
Dear Andrea
Thank you very much for your useful guidances.
Best regards,
Mitra