[QE-users] inaccurate LDA bandstructure compared to VASP in FeSe system

[Tiangang Zhou]

Hello all,

I'm working on the beta-FeSe system. In the first step I have done a simple
LDA calculation. Unfortunately, after adjusting ecutwfc and scf K_POINTS, I
can’t get the relatively accurate result compared to VASP and wien2k (the
result of VASP and wien2k agree well on the fermi surface). It’s likely
that the pseudopotential has some problem or I miss some key steps. The
input files are listed below.

fe_se.scf.in
```
&control
calculation = 'scf'
restart_mode='from_scratch',
prefix='fe_se',
pseudo_dir = '/home/tgzhou/softwares/qe_release_6.4/pseudo/',
outdir='tempdir/'
/
&system
ibrav=  6, celldm(1)= 7.124270293115536, celldm(3)= 10.431292329,
nat=  4, ntyp= 2,
ecutwfc = 120.0,
localization_thr = 0.01,
occupations='smearing', smearing='mp', degauss=0.02
/
&electrons
mixing_beta = 0.7,
conv_thr =  1.0d-10
/
ATOMIC_SPECIES
Fe  55.845  Fe.pz-n-kjpaw_psl.1.0.0.UPF
Se  78.96   Se.pz-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS crystal
Fe       0.000000000   0.000000000   0.000000000
Fe       0.500000000   0.500000000   0.000000000
Se       0.000000000   0.500000000   0.768115942
Se       0.500000000   0.000000000   0.231884057
K_POINTS automatic
8 8 4 0 0 0
```

fe_se.band.in
```
&control
calculation='bands'
pseudo_dir = '/home/tgzhou/softwares/qe_release_6.4/pseudo/',
outdir='tempdir/',
prefix='fe_se'
/
&system
ibrav=  6, celldm(1)= 7.124270293115536, celldm(3)= 10.431292329,
nat=  4, ntyp= 2,
ecutwfc = 120.0,
nbnd=20
/
&electrons
/
ATOMIC_SPECIES
Fe  55.845  Fe.pz-n-kjpaw_psl.1.0.0.UPF
Se  78.96   Se.pz-n-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS crystal
Fe       0.000000000   0.000000000   0.000000000
Fe       0.500000000   0.500000000   0.000000000
Se       0.000000000   0.500000000   0.768115942
Se       0.500000000   0.000000000   0.231884057
K_POINTS crystal_b
4
0.0 0.0 0.0 50
0.5 0.0 0.0 50
0.5 0.5 0.0 50
0.0 0.0 0.0 1
```


The band structure is in the path \Gamma-X-M-\Gamma. And those two have
some similarity but the qe FeSe band are much more narrow.

qe FeSe band structure
https://drive.google.com/file/d/1wG1Nm-f5v03-kfefJoPZ4gSD4rYaJSQa/view
vasp FeSe band structure
https://drive.google.com/file/d/1T0iNg7oecpey2dD6BB9iNrvjS6rm7Mnr/view


Best,

Tiangang Zhou
School of Physics
Peking University
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