[QE-users] Problem in Band Gap Calculation Using PBE with Different Pseudopotentials
Giovanni Cantele
giovanni.cantele at spin.cnr.it
Thu Mar 6 11:07:48 CET 2025
Hi,
maybe you should check your reference calculation, pay attention to the
used k-point grid, cutoff, lattice constant and so on,
because the risk is to compare calculations that are not made under the
same conditions.
This being said, the calculation can run quite easily on a PC, so I was
able to run it:
1) with the lattice constant you provided, the system is slightly metallic
(conduction band and valence band overlap in
a tiny energy region)
2) using a more appropriate lattice constant, and relaxing the atomic
positions, which I took from this paper
https://www.sciencedirect.com/science/article/pii/S0169433224002083
which uses the commercial package you mentioned, I get a very similar band
structure (Fig. 3d therein) and and energy gap
of 0.64 eV to be compared with 0.59 eV that can be obtained from the band
structure plot. The two gaps do not seem to differ so much.
I downloaded PAW pseudopotentials from the QE web site.
Here the input I used:
- geometry optimization
&CONTROL
calculation='relax'
title='naalo2 opt'
outdir='./tmp/'
prefix='pwscf'
pseudo_dir='.'
verbosity='high'
tprnfor=.true.
tstress=.true.
etot_conv_thr=1d-14
forc_conv_thr=1d-05
/
&SYSTEM
ibrav=4
a = 3.32
c = 20
nat=4
ntyp=3
ecutwfc=70
ecutrho=560
assume_isolated='2D'
occupations='smearing'
smearing='gaussian'
degauss=0.005d0
/
&ELECTRONS
conv_thr=1d-10
diago_full_acc = .true.
/
&IONS
/
ATOMIC_SPECIES
Na 22.98980000 Na.pbe-spn-kjpaw_psl.1.0.0.UPF
Al 26.98150000 Al.pbe-n-kjpaw_psl.1.0.0.UPF
O 15.99940000 O.pbe-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
Na 0.00000000000000 0.00000000000000 0.59729949500415
Al 0.66666666666667 0.33333333333333 0.44829871971219
O 0.66666666666667 0.33333333333333 0.53369883039369
O 0.33333333333333 0.66666666666667 0.42070295488990
K_POINTS {automatic}
8 8 1 0 0 0
- nscf for band structure calculation
same as above with
calculation='bands'
and
K_POINTS { crystal_b }
4
gG 50
M 50
K 50
gG 50
I would suggest to play with calculation parameters, to make a careful
lattice constant optimization, and so on, before
trying to obtain from your calculation numberd to be compared with previous
works.
Giovanni
--
Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it <giovanni.cantele at spin.cnr.it>
Phone: +39 081 676910
Skype contact: giocan74
ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: https://sites.google.com/view/giovanni-cantele/home
Il giorno mer 5 mar 2025 alle ore 20:35 Zahid Rashid via users <
users at lists.quantum-espresso.org> ha scritto:
> Dear QE users,
>
> I am trying to calculate the electronic band structure for 2D hexagonal
> NaAlO2 monolayers using Quantum Espresso 7.3. The band structure for this
> system has already been reported with a commercial package (VASP) and the
> band gap (reproducible) is ~ 0.35 eV (direct at Gamma point) using PBE
> functional and PAW pseudopotentials.
> Using Quantum Espresso-7.3, I tried to reproduce these results using PBE +
> PAW, PBEsol + PAW, PBE + Ultrasoft pseudopotentials, PBE + ONCVPSP
> (Optimized Norm-Conservinng Vanderbilt pseudopotential), PBE + NC
> (Troullier-Martins) PSlibrary 0.3.1 downloaded from
> https://theos-wiki.epfl.ch/en/Main/Pseudopotentials . All these
> pseudopotentials give a zero band gap. I tried a wave function cutoff value
> upto 150 Ry with ecutrho = 4 * ecutwfc .
> FHI PP with PBE from QE site gives a band gap of ~ 0.11 eV.
> Next I also calculated the band structure using NC PPs and HSE functional
> and I get a band gap (~ 1.27 eV) comparable to the reported value of ~1.2
> eV. My input for vc-relax is attached. The lattice constants reported with
> commercial package are a = b = 3.19 angstroms. I get a value between ~ 3.16
> to 3.188 angstroms with different pseudopotentials. Can you give some
> suggestions to reproduce the band gap with PBE functional for this system
> using QE package?
>
> &CONTROL
> calculation='vc-relax',
> title='naalo2 opt',
> outdir='.',
> prefix='pwscf',
> pseudo_dir='.',
> verbosity='high',
> tprnfor=.true.,
> tstress=.true.,
> etot_conv_thr=1d-14,
> forc_conv_thr=1d-05,
> /
>
> &SYSTEM
> ibrav=4,
> celldm(1) = 5.9704784375296835
> celldm(3) = 6.330233506939965
> nat=4,
> ntyp=3,
> ecutwfc=110,
> ecutrho=440,
> input_dft='pbe',
> assume_isolated='2D',
> occupations='smearing',
> smearing='gaussian',
> degauss=0.005d0,
> /
>
> &ELECTRONS
> conv_thr=1d-10,
> diagonalization='david'
> diago_full_acc = .true.
> /
>
> &IONS
> ion_dynamics='bfgs',
> /
>
> &CELL
> cell_dofree='2Dxy'
> cell_dynamics='bfgs',
> press=0.0d0,
> press_conv_thr=0.1d0,
> /
>
> ATOMIC_SPECIES
> Na 22.98980000 Na.pbe-n-nc.UPF
> Al 26.98150000 Al.pbe-n-nc.UPF
> O 15.99940000 O.pbe-nc.UPF
>
> ATOMIC_POSITIONS (crystal)
> Na 0.00000000000000 0.00000000000000
> 0.59729949500415
> Al 0.66666666666667 0.33333333333333
> 0.44829871971219
> O 0.66666666666667 0.33333333333333
> 0.53369883039369
> O 0.33333333333333 0.66666666666667
> 0.42070295488990
>
> K_POINTS {automatic}
> 25 25 1 0 0 0
>
> Regards.
>
> Zahid Rashid
> Department of Chemistry,
> Quaid-i-Azam University, Islamabad, Pakistan.
>
>
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