Our G0W0 results are obviously underestimated

[chinaye]

Dear all,
we performed G0W0 calculations starting form a PBE KS system for cubic ZnS. The direct band gaps are 2.00 (PBE), 2.77 (ppa), and 3.33 (cohsex) eV. Our G0W0 results are obviously underestimated with respect to other G0W0 ones reported in the literatures. For instance, the ppa G0W0 result is around 3.5 eV. We increased the density of the k-mesh and the number of the energy bands. However, the underestimate becomes even large. The same underestimate was found in the case of cubic ZnSe. Should we start our G0W0 calcultions from PBE+U KS systems? The following is the input file we used to carry out the G0W0 calculation. It will be appreciated very much if someone gives us some instructions. Thank you, very much.

input file
ndtset 3

# Definition of parameters for the calculation of the KSS file
nbandkss1 -1 # Number of bands in KSS file (-1 means the maximum possible)
nband1 75 # Number of (occ and empty) bands to be computed
kssform1 3
istwfk1 47*1

# Calculation of the screening (epsilon^-1 matrix)
optdriver2 3 # Screening calculation
getkss2 1 # Obtain KSS file from previous dataset
nband2 70 # Bands to be used in the screening calculation
npweps2 169
npwwfn2 169
#ppmfrq2 16.7 eV # Imaginary frequency where to calculate the screening
gwcalctyp2 = 9
#gwcomp2 1
#gwencomp2 1.0

# Calculation of the Self-Energy matrix elements (GW corrections)
optdriver3 4 # Self-Energy calculation
getkss3 1 # Obtain KSS file from dataset 1
getscr3 2 # Obtain SCR file from previous dataset
gwcalctyp3 = 8
nband3 70 # Bands to be used in the Self-Energy calculation
npwsigx3 169
npwwfn3 169
#gwcomp3 1
#gwencomp3 1.0

# Data common to the three different datasets

# Definition of the unit cell: fcc
acell 3*1 Angstrom

rprim -0.000000000000002 2.702649999999999 2.702650000000001
2.702649999999999 -0.000000000000001 2.702650000000001
2.702649999999999 2.702650000000000 0.000000000000001

# Definition of the atom types
ntypat 2
znucl 16 30

# Definition of the atoms
natom 2
typat 1 2
xred
0.25 0.25 0.25
0.00 0.00 0.00

# Definition of the k-point grid
kptopt 1 # Option for the automatic generation of k points,
nkpt 47
ngkpt 10 10 10

nshiftk 1
shiftk 0.0 0.0 0.0 # These shifts will be the same for all grids
istwfk 47*1 # This is mandatory in all the GW steps.

# Use only symmorphic operations
symmorphi 0

ecut 35.0
# Definition of the SCF procedure
nstep 10000
ixc 11
toldfe 1.0d-8
diemac 12.0

#Common to all model GW calculations
nkptgw 1
kptgw
0.00000000E+00 0.00000000E+00 0.00000000E+00
bdgw
9 10

# This line added when defaults were changed (v5.3) to keep the previous, old behaviour
iscf 5

[bruneval]

Does your pseudopotential for Zn consider 3s and 3p electrons as valence ? This is of the utmost importance for the final result.

It has been published several times that G0W0 for ZnO, ZnS, and ZnSe underestimates quite much the experimental band gap. You may turn to self-consistent GW to improve the agreement.

Good luck.

Fabien