GW accurancy

[Newlife1982]

Hi all,

I do the GW calculations about crystal Si. The band gap is about 1.34 eV, which is not very good compared with the experimental value 1.17 eV and others calculated value 1.24 eV.
I increase the cutoff energy and use different gwcalctyp methods as well as different exchange functional. But the calculated values are also bigger than 1.34. I do not know what is wrong
with my calculations. Anybody know about it? It would be very nice if you could give me some advices. Thank you very much.

[bruneval]

1.34 eV is not that bad.

Difficult to help you without your input files...

Fabien

[Newlife1982]

The input files are as follows. Thank you in advance for your reply.

# Create the KSS file

kssform 1
nbandkss 100

nband 100
prtwf 0

#Definition of occupation numbers
occopt 1
tsmear 0.01

#Definition of the unit cell
acell 3*10.259

rprim 0.0 0.5 0.5 # FCC primitive vectors (to be scaled by acell)
0.5 0.0 0.5
0.5 0.5 0.0

#Definition of the atom types
ntypat 1
znucl 14

#Definition of the atoms
natom 2
typat 1 1
xred 0.0 0.0 0.0
0.25 1.25 0.25

#Definition of the planewave basis set
ecut 20.0

#Definition of the k-point grid
ngkpt 4 4 4

#64 k points
nshiftk 1
shiftk 0. 0. 0.
istwfk 8*1
#256 k points
#nshiftk 4
#shiftk 0 0 0 1/2 1/2 0 1/2 0 1/2 0 1/2 1/2
#istwfk 19*1

#Definition of the SCF procedure
nstep 50
toldfe 1.0d-8

prtvol 5
enunit 1


.............................................................................................................................................

# create the screening file

#Parameter for the screening calculation
optdriver 3
gwcalctyp 28
nband 100
ecuteps 17.0
ecutwfn 17.0

nfreqim 4
nfreqre 10
freqremax 1.
awtr 0 # Note : the default awtr 1 is better

#Definition of occupation numbers
occopt 1
tsmear 0.05

#Definition of the unit cell
acell 3*10.259

rprim 0.0 0.5 0.5 # FCC primitive vectors (to be scaled by acell)
0.5 0.0 0.5
0.5 0.5 0.0

#Definition of the atom types
ntypat 1
znucl 14

#Definition of the atoms
natom 2
typat 1 1
xred 0.0 0.0 0.0
0.25 1.25 0.25

#Definition of the planewave basis set
ecut 20.0

#Definition of the k-point grid
ngkpt 4 4 4
nshiftk 1
shiftk 0. 0. 0.
istwfk 8*1

#Definition of the SCF procedure
nstep 50
toldfe 1.0d-8

prtvol 5
enunit 1

........................................................................................................................................

calculation of the quasi-particle Fermi energy

#Parameters for the GW calculation
optdriver 4
nband 100
ecutsigx 17
ecutwfn 17

gwcalctyp 28

nkptgw 8
kptgw
0.000000 0.000000 0.000000
0.250000 0.000000 0.000000
0.500000 0.000000 0.000000
0.250000 0.250000 0.000000
0.500000 0.250000 0.000000
-0.250000 0.250000 0.000000
0.500000 0.500000 0.000000
-0.250000 0.500000 0.250000
bdgw
1 50
1 50
1 50
1 50
1 50
1 50
1 50
1 50

#Definition of occupation numbers
occopt 1
tsmear 0.01

#Definition of the unit cell
acell 3*10.259

rprim 0.0 0.5 0.5 # FCC primitive vectors (to be scaled by acell)
0.5 0.0 0.5
0.5 0.5 0.0

#Definition of the atom types
ntypat 1
znucl 14

#Definition of the atoms
natom 2
typat 1 1
xred 0.0 0.0 0.0
0.25 1.25 0.25
#Definition of the planewave basis set
ecut 20.0
#Definition of the k-point grid
ngkpt 4 4 4
nshiftk 1
shiftk 0. 0. 0.
istwfk 8*1

#Definition of the SCF procedure
nstep 50
toldfe 1.0d-8

prtvol 5
enunit 1

[bruneval]

Your k-point set appears a bit too small. Try to increase it.

Fabien

[Newlife1982]

Fabien:

Thank you very much. I am trying the new K-point.

Wang

[Steve]

You really use 1.25?
xred 0.0 0.0 0.0
0.25 1.25 0.25
Best regards, Steve.