SnS band gap is not matching with experiment

[chowdhury.cua]

Dear All,

I am working on bulk SnS. Its indirect band gap is 1.05 eV. Using DFT I am getting 0.82 eV. So I am calculating the indirect band gap using Yambo. I am getting it is 0.86 eV. Below I have mentioned my PWSCF input and Yambo inputs, please find. Please let me know where I am making mistakes.

Thank you for your help and time.

Best,
Sugata

PWSCF SCF:-
&CONTROL
calculation="scf",
restart_mode='from_scratch',
outdir = '/tmp/SnS_VC_Bulk_SRL/'
prefix = 'SnS_VC_Bulk_SRL',
pseudo_dir ='/home/snc8/2D-Materials/Potential/PBE/Norm',
forc_conv_thr = 1.0D-4,
etot_conv_thr = 1.0D-3,
!tstress=.true.
!tprnfor=.true.
wf_collect=.true.
verbosity = 'high'
nstep = 5000

/
&SYSTEM
ibrav = 0,
celldm(1) = 21.164931072
!celldm(2) = 0.35625,
!celldm(3) = 0.3875,
nat = 8,
ntyp = 2,
!nbnd = 120,
lspinorb = .true.,
noncolin = .true.,
ecutwfc = 50.D0,
ecutrho = 300.D0,
force_symmorphic=.true.
nosym = .false.
occupations='smearing',
smearing='marzari-vanderbilt',
degauss=0.02

/
&ELECTRONS
diago_full_acc=.true.,
diago_thr_init = 1.0d-6,
!electron_maxstep = 100
!startingwfc='atomic'
!diagonalization='david'
!mixing_mode = 'local-TF'
mixing_beta = 0.5,
!mixing_ndim = 8,
conv_thr = 1.0D-8

/
!&IONS
! upscale = 100

/
!&CELL
! cell_factor = 1.8D0

/
CELL_PARAMETERS {alat}
1.00000000 0.00000 0.0000
0.00000000 0.35625 0.0000
0.00000000 0.00000 0.3875

ATOMIC_SPECIES
Sn 118.71 sn_frl_gga.upf
S 32.065 s_frl_gga.upf

K_POINTS {automatic}
8 8 8 0 0 0

ATOMIC_POSITIONS {crystal}
Sn 0.117626548 0.250000000 0.116504282
Sn 0.882373452 0.750000000 0.883495718
Sn 0.382373452 0.750000000 0.616504282
Sn 0.617626548 0.250000000 0.383495718
S 0.853687919 0.250000000 0.469697643
S 0.146312081 0.750000000 0.530302357
S 0.646312081 0.750000000 0.969697643
S 0.353687919 0.250000000 0.030302357

PWSCF NSCF:-
&CONTROL
calculation="nscf",
restart_mode='from_scratch',
outdir = '/tmp/SnS_VC_Bulk_SRL/'
prefix = 'SnS_VC_Bulk_SRL',
pseudo_dir ='/home/snc8/2D-Materials/Potential/PBE/Norm',
forc_conv_thr = 1.0D-5,
etot_conv_thr = 1.0D-6,
tstress=.true.
tprnfor=.true.
wf_collect=.true.
verbosity = 'high'
nstep = 5000

/
&SYSTEM
ibrav = 0,
celldm(1) = 21.164931072
!celldm(2) = 0.35625,
!celldm(3) = 0.3875,
nat = 8,
ntyp = 2,
nbnd = 140,
lspinorb = .true.,
!nspin = 2
!constrained_magnetization = 'total'
!starting_magnetization = 0.0,
!fixed_magnetization(1) = 0.000001,
!tot_magnetization = 0
noncolin = .true.,
!starting_magnetization(1) = 1.0,
ecutwfc = 50.D0,
!ecutrho = 300.D0,
force_symmorphic=.true.
nosym = .true.
occupations='smearing',
smearing='marzari-vanderbilt',
degauss=0.02

/
&ELECTRONS
electron_maxstep = 100
startingwfc='atomic'
diagonalization='david'
mixing_mode = 'local-TF'
mixing_beta = 0.5,
!mixing_ndim = 8,
conv_thr = 1.0D-8

/
&IONS
upscale = 100

/
&CELL
cell_factor = 1.8D0

/
CELL_PARAMETERS {alat}
1.00000000 0.00000 0.0000
0.00000000 0.35625 0.0000
0.00000000 0.00000 0.3875
ATOMIC_SPECIES
Sn 118.71 sn_frl_gga.upf
S 32.065 s_frl_gga.upf

ATOMIC_POSITIONS {crystal}
Sn 0.117626548 0.250000000 0.116504282
Sn 0.882373452 0.750000000 0.883495718
Sn 0.382373452 0.750000000 0.616504282
Sn 0.617626548 0.250000000 0.383495718
S 0.853687919 0.250000000 0.469697643
S 0.146312081 0.750000000 0.530302357
S 0.646312081 0.750000000 0.969697643
S 0.353687919 0.250000000 0.030302357


K_POINTS {crystal}
256
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Yambo Input:-
ppa # [R Xp] Plasmon Pole Approximation
gw0 # [R GW] GoWo Quasiparticle energy levels
HF_and_locXC # [R XX] Hartree-Fock Self-energy and Vxc
em1d # [R Xd] Dynamical Inverse Dielectric Matrix
FFTGvecs= 9843 RL # [FFT] Plane-waves
X_all_q_CPU= "4 4 4 4" # [PARALLEL] CPUs for each role
X_all_q_ROLEs= "q k c v" # [PARALLEL] CPUs roles (q,k,c,v)
X_all_q_nCPU_invert=0 # [PARALLEL] CPUs for matrix inversion
SE_CPU= "4 4 4" # [PARALLEL] CPUs for each role
SE_ROLEs= "q qp b" # [PARALLEL] CPUs roles (q,qp,b)
EXXRLvcs= 62483 RL # [XX] Exchange RL components
Chimod= "" # [X] IP/Hartree/ALDA/LRC/BSfxc
% QpntsRXp
1 | 260 | # [Xp] Transferred momenta
%
% BndsRnXp
1 | 120 | # [Xp] Polarization function bands
%
NGsBlkXp= 1 RL # [Xp] Response block size
% LongDrXp
1.000000 | 0.000000 | 0.000000 | # [Xp] [cc] Electric Field
%
PPAPntXp= 27.21138 eV # [Xp] PPA imaginary energy
% GbndRnge
1 | 120 | # [GW] G[W] bands range
%
GDamping= 0.10000 eV # [GW] G[W] damping
dScStep= 0.10000 eV # [GW] Energy step to evalute Z factors
GTermKind= "none" # [GW] GW terminator ("none","BG" Bruneval-Gonze,"BRS" Berger-Reining-Sottile)
DysSolver= "n" # [GW] Dyson Equation solver (`n`,`s`,`g`)
%QPkrange # [GW] QP generalized Kpoint/Band indices
1|260| 1|120|
%
%QPerange # [GW] QP generalized Kpoint/Energy indices
1|260| 0.0|-1.0|
%

[amolina]

Dear Sugata,

in principle, it is difficult to say just from the input why the GW correction does not match the experimental value. There could be many reasons. In order to be sure that the GW correction is well done, I would recommend to make a convergence study of the yambo input variables like FFTGvecs, GbndRnge, NGsBlkXp, EXXRLvcs, BndsRnXp and of course, QpntsRXp. The values for the number of bands can be quite high and so the number of k-points (although in your calculations you are using quite a lot of k-points). Also you are using NGsBlkXp=1. In anisotropic materials you could increase it. Another is the plasmon pole approx. Can you apply that approx. in your material?

Kind regards,

Alejandro.

[chowdhury.cua]

Dear Alejandro,

Thank you for your quick reply. I will follow your suggestions (convergence study). I will get back to you by tomorrow.

Best,
Sugata