[QE-users] Wrong position of fermi level in bulk Cr2O3

Nicola Marzari nicola.marzari at epfl.ch
Thu Jul 23 14:38:22 CEST 2020



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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