Wrong VBM and CBM in mobility calculation
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sdwang
Joined: Mon Dec 26, 2016 10:49 am Posts: 22
University: Inner Mongolian Univ
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 Wrong VBM and CBM in mobility calculation
Dear all, I ran the mobility calculation and found the VBM and CBM in the epw2.out are not consistent with nscf.out values. In the epw2.out I found that:
...skipping... Fermi energy coarse grid = 0.000000 eV
===================================================================
Fermi energy is read from the input file: Ef = 4.539000 eV
===================================================================
ibndmin = 10 ebndmin = 0.040 ibndmax = 17 ebndmax = 0.628
Number of ep-matrix elements per pool : 2239488 ~= 17.09 Mb (@ 8 bytes/ DP) Applying a scissor shift of 1.50000 eV to the conduction states
Temperature 300.000 K Valence band maximum = 8.848697 eV Conduction band minimum = 8.359119 eV Warning: too many iterations in bisection Ef = 8.359119 Mobility VB Fermi level 8.359119 eV Valence band maximum = 8.848697 eV Conduction band minimum = 8.359119 eV Warning: too many iterations in bisection Ef = 8.848697 Mobility CB Fermi level 8.848697 eV
=================================================================== Scattering rate =================================================================== where the VBM and CBM are far larger than in the nscf.out , where they are 4.539 and 6.6407. My in put of epw2.in is: &inputepw prefix = 'tc' amass(1) = 12.01078, !amass(2) = 32.07, outdir = './'
iverbosity = 0
elph = .true. kmaps = .true. epbwrite = .false. epbread = .false. epwwrite = .false. epwread = .true. etf_mem = 1
scattering = .true. scattering_serta = .true. int_mob = .true. carrier = .true. ncarrier = 1E13
scissor = 1.5 nstemp = 1 tempsmin = 300 tempsmax = 300
restart = .true. restart_freq = 100
nbndsub = 20 nbndskip = 0 lifc = .true. asr_typ = 'crystal'
wannierize = .false. fsthick = 4.0 ! eptemp = 300.00 ! K (same as PRB 76, 165108) degaussw = 0.015 ! eV efermi_read = .true. fermi_energy= 4.5390 dvscf_dir = '../phonon/save' !filkf = './inp.kpt' !filqf = './inp.kpt'
nkf1 = 36 nkf2 = 36 nkf3 = 36
nqf1 = 36 nqf2 = 36 nqf3 = 36 nk1 = 12 nk2 = 12 nk3 = 12
nq1 = 6 nq2 = 6 nq3 = 6 / 32 cartesian ...
Thanks!
SD
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Mon Jan 07, 2019 3:52 pm |
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sponce
Site Admin
Joined: Wed Jan 13, 2016 7:25 pm Posts: 575
University: Oxford
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 Re: Wrong VBM and CBM in mobility calculation
Dear sdwang,
You should put the Fermi energy in the middle of your gap not at the VBM. So around 5.5 eV Now if the gap is too large, it might be difficult for the bisection method to converge (deals with huge exponentials). Therefore I would remove the scissor = 1.5 from the calculation. Indeed the mobility will not be affected. The scissor is typically used if the DFT bandgap is very small.
Note that the bisection algo has been improved. You can try the most recent GitLab developer version. In any case there will be a new release of QE and EPW at the end of Feb 2019.
Best wishes, Samuel
_________________ Dr. Samuel Poncé Department of Materials University of Oxford Parks Road Oxford OX1 3PH, UK
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Fri Jan 25, 2019 12:50 pm |
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sdwang
Joined: Mon Dec 26, 2016 10:49 am Posts: 22
University: Inner Mongolian Univ
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 Re: Wrong VBM and CBM in mobility calculation
Dear Samuel, Thank you for the reply. I have removed the scissor and fixed the Fermi level in the middle o the gap, and the CBM seems OK but the VBM is still high than the VBM(like below). The band structure of the interpolated with EPW agrees well with DFT one.
Temperature 300.000 K Valence band maximum = 8.846493 eV Conduction band minimum = 6.859119 eV Warning: too many iterations in bisection Ef = 6.859119 Mobility VB Fermi level 6.859119 eV Valence band maximum = 8.846493 eV Conduction band minimum = 6.859119 eV Warning: too many iterations in bisection Ef = 8.846493 Mobility CB Fermi level 8.846493 eV
=================================================================== Scattering rate ===================================================================
Fermi Surface thickness = 4.500000 eV This is computed with respect to the fine Fermi level 5.558700 eV Only states between 1.058700 eV and 10.058700 eV will be included ... Thanks in advance!
Best
SD
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Sun Jan 27, 2019 12:35 am |
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sponce
Site Admin
Joined: Wed Jan 13, 2016 7:25 pm Posts: 575
University: Oxford
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 Re: Wrong VBM and CBM in mobility calculation
Dear SD, If you still get the warning in the bissection method, then you cannot expect correct results. Have you tried the development version https://gitlab.com/QEF/q-e Best wishes, Samuel
_________________ Dr. Samuel Poncé Department of Materials University of Oxford Parks Road Oxford OX1 3PH, UK
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Sun Jan 27, 2019 1:23 pm |
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sdwang
Joined: Mon Dec 26, 2016 10:49 am Posts: 22
University: Inner Mongolian Univ
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 Re: Wrong VBM and CBM in mobility calculation
Dear Samuel, I'v installed the dev version of QE, and I will try the new EPW to see waht's goning on then... I noticed in the test-suite there is a direotry with a USPP for EPW. Does this mean we can deal the system with USPP in the new version of EPW? Thanks!
Shudong
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Mon Jan 28, 2019 3:23 am |
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sponce
Site Admin
Joined: Wed Jan 13, 2016 7:25 pm Posts: 575
University: Oxford
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 Re: Wrong VBM and CBM in mobility calculation
Dear sdwang, It is very new and therefore not very well tested but yes, in principle USPP should now be supported  Best, Samuel
_________________ Dr. Samuel Poncé Department of Materials University of Oxford Parks Road Oxford OX1 3PH, UK
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Mon Jan 28, 2019 10:06 am |
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piotr
Joined: Wed Dec 26, 2018 8:57 am Posts: 5
University: IAP RAS
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 Re: Wrong VBM and CBM in mobility calculation
Dear, all I've got similar problem, but in my case increasing number of q-points (from 6x6x6 to 8x8x8) helps. If plot interpolated bands on the coarser grid than there is an artifact near the VBM that causes the issue. Here mesh is 12x12x12, so probably it is not the case, but plotting interpolated bands may be helpful anyway.
Best wishes, Petr
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Tue Jan 29, 2019 3:51 pm |
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sdwang
Joined: Mon Dec 26, 2016 10:49 am Posts: 22
University: Inner Mongolian Univ
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 Re: Wrong VBM and CBM in mobility calculation
Dear Petr, Thanks for the advice. My interpolated bands agree well with the DFT one for 12x12x12 k-point. Anyway, I would increase the q-point to check the results.
Best,
Shudong
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Wed Jan 30, 2019 1:22 am |
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sponce
Site Admin
Joined: Wed Jan 13, 2016 7:25 pm Posts: 575
University: Oxford
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 Re: Wrong VBM and CBM in mobility calculation
Are you sure that "nbndskip = 0" is correct in your case.
How many electrons do you have in your nscf.in calculations ? With respect to your nscf calculation, are you skipping bands or not ? If so you have to tell EPW. This typically will lead to wrong VBM and CBM determination.
Best wishes, Samuel
_________________ Dr. Samuel Poncé Department of Materials University of Oxford Parks Road Oxford OX1 3PH, UK
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Thu Feb 14, 2019 2:14 pm |
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sdwang
Joined: Mon Dec 26, 2016 10:49 am Posts: 22
University: Inner Mongolian Univ
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 Re: Wrong VBM and CBM in mobility calculation
Dear Samuel, My nscf file contains 32 electrons and nbnd=32 in the input, and I set nbndsub = 20 , nbndskip = 0 in the epw.in. In this case, How do I tell EPW the 'real' band setting in epw.in file? Does it matter with fsthick = 4.0 since I included 20 nbndsub? Thanks!
Best
Shudong
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Fri Feb 15, 2019 12:34 am |
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