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

[Pietro Delugas]

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 for Windows 10

From: Yuvam Bhateja
Sent: Thursday, July 23, 2020 12:44 PM
To: Quantum ESPRESSO users Forum
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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