[QE-users] Why is the Dirac point of the pristine graphene above the Fermi level through the calculation with Quantum Espresso?

[Lorenzo Paulatto]

Dear 宋克楠,

your grid+supercell in incommensurate with the Dirac point, so its 
position in energy is not constrained, using a grid which a multiple of 
3 may fix the issue, however there could be another cause. Your cell has 
a lot of vacuum space, with the PBE functional, you sometimes get a 
wandering electron which ruins the band structure, reducing the vacuum 
space to about 7Å maximum would fix it.

hth

On 02/09/2024 02:54, 宋克楠 wrote:
> Dear All,
>
> I am running a test calculation for the band structure of the pristine 
> monolayer graphene with 4×4×1
> supercell through Quantum Espresso (QE).
>
> This is my input file.
>
> &CONTROL
>   title = 'SLG_441',
>   calculation = 'scf',
>   restart_mode = 'from_scratch',
>   pseudo_dir = './'
> /
> &SYSTEM
>   ibrav = 0,
>   nat = 32,
>   ntyp = 1,
>   ecutwfc = 50,
>   ecutrho = 200,
>   nbnd = 156,
>   occupations = 'smearing',
>   degauss = 1d-9,
>   smearing = 'gaussian'
> /
> &ELECTRONS
>   electron_maxstep = 100,
>   conv_thr = 1.0d-9,
>   mixing_mode = 'plain',
>   mixing_beta = 0.8d0,
>   diagonalization = 'david'
> /
> &IONS
>     ion_positions = 'from_input'
> /
> &CELL
>     cell_dynamics = 'none'
> /
> ATOMIC_SPECIES
>   C  12.0 C.pbe-n-kjpaw_psl.1.0.0.UPF
> CELL_PARAMETERS (angstrom)
>   9.8400000000      0.0000000000     0.0000000000
>   -4.9200000000      8.5216901054     0.0000000000
>   0.0000000000      0.0000000000   23.4000000000
> ATOMIC_POSITIONS (angstrom)
> C   0.0000000000      0.0000000000     11.7000000000
> C   1.2300000367      0.7101408633     11.7000000000
> C  -1.2300000000      2.1304225263     11.7000000000
> C   0.0000000367      2.8405633896     11.7000000000
> C  -2.4600000000      4.2608450527     11.7000000000
> C  -1.2299999633      4.9709859160     11.7000000000
> C  -3.6900000000      6.3912675790     11.7000000000
> C  -2.4599999633      7.1014084423     11.7000000000
> C   2.4600000000      0.0000000000     11.7000000000
> C   3.6900000367      0.7101408633     11.7000000000
> C   1.2300000000      2.1304225263     11.7000000000
> C   2.4600000367      2.8405633896     11.7000000000
> C   0.0000000000      4.2608450527     11.7000000000
> C   1.2300000367      4.9709859160     11.7000000000
> C  -1.2300000000      6.3912675790     11.7000000000
> C   0.0000000367      7.1014084423     11.7000000000
> C   4.9200000000      0.0000000000     11.7000000000
> C   6.1500000367      0.7101408633     11.7000000000
> C   3.6900000000      2.1304225263     11.7000000000
> C   4.9200000367      2.8405633896     11.7000000000
> C   2.4600000000      4.2608450527     11.7000000000
> C   3.6900000367      4.9709859160     11.7000000000
> C   1.2300000000      6.3912675790     11.7000000000
> C   2.4600000367      7.1014084423     11.7000000000
> C   7.3800000000      0.0000000000     11.7000000000
> C   8.6100000367      0.7101408633     11.7000000000
> C   6.1500000000      2.1304225263     11.7000000000
> C   7.3800000367      2.8405633896     11.7000000000
> C   4.9200000000      4.2608450527     11.7000000000
> C   6.1500000367      4.9709859160     11.7000000000
> C   3.6900000000      6.3912675790     11.7000000000
> C   4.9200000367      7.1014084423     11.7000000000
> K_POINTS automatic
> 11 11 1 1 1 1
>
> I am using C.pbe-n-kjpaw_psl.1.0.0.UPF pseudopotential with the 
> details below.
>
> Origin: PS Library
> Author: ADC
> Generated using "atomic" code by A. Dal Corso  v.6.3
> Pseudopotential type: PAW
> Functional type: PBE
> Non Linear Core Correction
> Scalar relativistic
>
> I found that the Dirac point is located 0.027 eV above the Fermi 
> energy level, shown in the figure blow. Obviously, it is not correct.
>
> Is the problem from the pseudopotential?
>
> Would anyone please give me some suggestions on my input file and 
> recommend a proper pseudopotential to me?
>
> Thank you in advance.
>
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-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
phone: +33 (0)1 442 79822 / skype: paulatz
http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
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