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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