[QE-users] Wrong position of fermi level in bulk Cr2O3
Pietro Delugas
pdelugas at sissa.it
Fri Jul 24 09:13:17 CEST 2020
I am not sure I have understood what the problem is.
If all you need is the plot of the density of states and want to set in the plot the position of the Fermi level, my suggestion, this being an insulator, is to avoid to indicate a value for the Fermi level, just put a VBT ( valence band top) in correspondence of the HOMO and a CBM ( conduction band minimum ) in correspondence of the LUMO.
Usually if you are more interested in the behavior of the holes ( p-type) condition you shift the plot in order that VBT is at 0, or if you are more interested on electrons (n-type) in conduction band you shift the plot in order to have the CBM at 0.
If you are interested in both and want to indicate the Fermi Level you can only indicate the intrinsic one which is in the middle of the gap between VBT and CBM.
HTH
regards
Pietro
Sent from Mail for Windows 10
From: Yuvam Bhateja
Sent: Thursday, July 23, 2020 6:01 PM
To: Quantum ESPRESSO users Forum
Cc: Pietro Delugas
Subject: Re: [QE-users] Wrong position of fermi level in bulk Cr2O3
Thank you for the reply.
I am not sure how to solve this problem.
I tried using different pseudopotentials with diffrent XC. Even tried increasing U value.
In most of the papers, pure bulk Cr2O3 is used.
Can you please advise me?
Thank you in advance.
Regards
Yuvam
On Thu, 23 Jul 2020, 6:08 pm Nicola Marzari, <nicola.marzari at epfl.ch> wrote:
Indeed! Although I would feel better about using as Fermi energy the
zero temperature limit of the chemical potential, that would be
(homo+lumo)/2? (well, the band edges).
nicola
PS: of course the lumo is wrong even in exact DFT, and the homo is
correct only in exact DFT, and wrong in approximate DFT.
On 23/07/2020 14:35, Pietro Delugas wrote:
> Dear Yuvam
>
> When one uses smeared occupations in systems with a large gap (
> significantly larger than the smearing) the final value of the Fermi
> level returned from the calculation may be any value within the band gap
> sufficiently far from the band edges (HOMO and LUMO). All the values
> in this range yield the same values for the occupations and thus of the
> ground state density. If you wish you can decide to follow the
> convention of the other papers and refer to the HOMO as the Fermi level.
>
> In fact in insulators the thermodynamic Fermi level depends on extrinsic
> conditions such as doping or defect concentrations and on the
> temperature. All things which are obviously not taken into account in
> your calculation.
>
> Kind regards
>
> Pietro
>
> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows 10
>
> *From: *Yuvam Bhateja <mailto:yuvamb16 at gmail.com>
> *Sent: *Thursday, July 23, 2020 12:44 PM
> *To: *Quantum ESPRESSO users Forum <mailto:users at lists.quantum-espresso.org>
> *Subject: *[QE-users] Wrong position of fermi level in bulk Cr2O3
>
> Hello dear experts,
>
> Hope you are doing well.
>
> I was trying to calculate the DOS in hexagonal Cr2O3, the DOS is coming
> very well with a bandgap of 3.09, but the problem is the Fermi level is
> at the edge of LUMO whereas I've observed that it should be at HUMO edge
> in few already published papers.
>
> I used PBE NC SR pseudopotential and also optimized with the same. I
> used gaussian smearing with a width of 0.001 in VC-relax and tetrahedra
> in SCF calculation.
>
> Here is my input file -
>
> &CONTROL
> calculation = "scf"
> pseudo_dir = "./"
> outdir = "./outdir"
> /
>
> &SYSTEM
> a = 4.6166825e+00
> c = 1.2457778e+01
> ecutrho = 4.5000e+02
> ecutwfc = 4.5000e+01
> ibrav = 4
> nat = 30
> nspin = 2
> ntyp = 3
> nbnd = 276
> starting_magnetization(1) = 0.5
> starting_magnetization(2) = -0.5
> starting_magnetization(3) = 0.0
> tot_magnetization = 0.0
> lda_plus_u = .true.
> Hubbard_U(1) = 4.7
> Hubbard_U(2) = 4.7
> Hubbard_J0(1) = 1
> Hubbard_J0(2) = 1
> /
>
> &ELECTRONS
> conv_thr = 1.00000e-11
> electron_maxstep = 2000
> mixing_beta = 7.00000e-01
> startingpot = "atomic"
> startingwfc = "atomic+random"
> /
>
>
> K_POINTS {automatic}
> 12 12 12 0 0 0
>
> ATOMIC_SPECIES
> Cr1 51.99610 Cr.upf
> Cr2 51.99610 Cr.upf
> O 15.99940 O.upf
>
> ATOMIC_POSITIONS (angstrom)
> Cr2 -0.0000027528 2.6654550870 3.9638155751
> Cr1 2.3083528721 1.3327252495 2.2651020880
> Cr1 -0.0000027528 2.6654550870 0.1887989665
> Cr2 0.0000000000 0.0000000000 1.8875092202
> Cr2 0.0000000000 0.0000000000 8.1164268017
> Cr1 -0.0000027528 2.6654550870 6.4177165986
> Cr1 0.0000000000 -0.0000000000 4.3414084067
> Cr2 2.3083528721 1.3327252495 6.0401185402
> Cr2 2.3083528721 1.3327252495 12.2690363475
> Cr1 0.0000000000 0.0000000000 10.5703258606
> Cr1 2.3083528721 1.3327252495 8.4940197683
> Cr2 -0.0000027528 2.6654550870 10.1927331413
> O 1.4445126756 0.0000000398 3.1144588851
> O 3.0306076646 0.0817435268 1.0381518347
> O 0.8638388763 1.3327251470 1.0381518347
> O 3.0306056520 2.5837116627 1.0381518347
> O -0.7222568310 1.2509851988 3.1144588851
> O 1.5860942747 2.7471960155 3.1144588851
> O 3.7528623252 1.3327254382 7.2670694393
> O 0.7222547656 1.4144695631 5.1907658203
> O -1.4445122603 2.6654550733 5.1907658203
> O 0.7222574947 3.9164369542 5.1907658203
> O 1.5860961469 2.5837115564 7.2670694393
> O 1.5860937210 0.0817433419 7.2670694393
> O 1.4445122976 2.6654551364 11.4196834856
> O -1.5860942579 2.7471959172 9.3433763434
> O 0.8638382678 3.9981804108 9.3433763434
> O 0.7222569078 1.2509852624 9.3433763434
> O -0.7222575680 3.9164369549 11.4196834856
> O -0.7222547296 1.4144694992 11.4196834856
>
> I later tried with pw91 USPP, it fixed the fermi level position (it's
> now at HUMO edge) but the bad gap is now very underestimated to 2.3 eV.
>
> I tried paw but it wasn't working with me.
>
> Can someone please help?
>
> Regards
>
> Yuvam Bhateja
>
> IIEST Shibpur
>
> India
>
>
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--
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project
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