I would think that band gap calculations are the most easy thing to do,
but I agree, it might be necessary to "think".
The problem could be that the default scf k-mesh, which could be shifted
in certain symmetries, may NEVER yield the correct band gap. If you are
interested in the gap, do NOT shift the k-mesh when running x kgen
(maybe 90 % of the compounds have their gap at Gamma)
Instructions for band gaps:
Initialization
x kgen (use unshifted mesh, since in most cases the gap is at Gamma)
scf cycle.
Check in scf file if there is a gap (:GAP)
Plot a band structure in various directions. Check the gap and in
particular if the gap is at Gamma or at some other k-point; if it is
direct or indirect. (Eigenvalues are listed in case.output1 in Ry, so
you should be able to calculate the gap exactly once you know which
k-points to check).
Compare your value with :GAP in the scf file.
If it agrees, ok.
If not: is the gap at Gamma and you have used a "shifted" scf k-mesh ???
Such a mesh does not include Gamma, and all "convergence tests of
k-meshes" ??? will fail, since even with 100000 k-points, still the gap
at Gamma is NOT included in the scf.
Am 23.12.2019 um 09:33 schrieb Gavin Abo:
I don't recall seeing "test_conv-5.0.tar.gz" before. I'm guessing that
is the newer version of the full convergence tests package at [1]
(unsupported at least by this group), which seems to not be accessible
to the public (and likely only accessible to and perhaps supported by
the private group [2]).
I'm guessing the algorithm that gets the gap value from the valence band
maximum (VBM) and conduction band minimum (CBM) is not accurate for your
calculation case. Therefore, I would most likely go with the gap that
you get yourself from the raw data (case.spaghetti_ene [3] or [4]) of
the band structure plot from spaghetti.
If the differences are small like in [5], a percent difference [6] of
about 2% [= (1.54 eV - 1.51 eV)/{(1.54 eV + 1.51 eV)/2}*100%] seems
acceptable. I believe greater than about 5% in when you should be
concerned [7].
[1]
http://wien2k-algerien1970.blogspot.com/2016/09/the-full-version-of-conversgence-tests.html
[2] https://www.facebook.com/groups/wien2k.Algeria/
[3]
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg15255.html
[4]
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg15258.html
[5]
https://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/msg19405.html
[6]
https://www.webassign.net/question_assets/ncsucalcphysmechl3/percent_error/manual.html
[7] https://socratic.org/questions/what-percent-error-is-too-high
On 12/22/2019 11:36 PM, Peeyush kumar kamlesh wrote:
Dear Gavin Sir,
Thank you for your response.
I did k-point convergence test by a test package
"test_conv-5.0.tar.gz". I dont know whether it is gives completely
satisfactory results or not. If you have any other package for kpt
convergence kindly let me know, so that i can do the right
calculations. Also I used non shifted k-mash in the calculations of
the band gap.
Thank you so much!
Regards
Peeyush Kamlesh
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