Graphene is a special material, you cannot just compute epsilon like
that. It is officially a metal, hence its static epsilon is infinity.
You should see that the better you do your calculation (i.e. more
k-points), the higher value you get.
Furthermore, its electric permittivity is strongly frequency dependent:
you would need some frequency-aware method to compute it. But I would
recommend you start with anything else instead.
cheers
On 9/16/19 11:55 PM, Jay Wai wrote:
Dear users,
I am trying to calculate dielectric constants of graphene.
I first relaxed the structure using the following pw.x input:
&CONTROL
calculation = 'relax'
tstress = .true.
tprnfor = .true.
prefix = 'noiso'
forc_conv_thr = 1.d-4
/
&SYSTEM
ibrav = 0
ecutwfc = 64
ecutrho = 576
ntyp = 1
nat = 8
nosym = .true.
/
&ELECTRONS
conv_thr=1.d-12
/
ATOMIC_SPECIES
C 12.0 C.pbe-n-rrkjus.UPF
K_POINTS automatic
12 12 1 0 0 0
CELL_PARAMETERS angstrom
2.45951214674781 4.26000000000000 0.00000000000000
-2.45951214674781 4.26000000000000 0.00000000000000
0.00000000000000 0.00000000000000 30.00000000000000
ATOMIC_POSITIONS angstrom
C 0.000000000 1.420000000 15.000000000
C 0.000000000 2.840000000 15.000000000
C -1.229756073 3.550000000 15.000000000
C -1.229756073 4.970000000 15.000000000
C 1.229756073 3.550000000 15.000000000
C 1.229756073 4.970000000 15.000000000
C 0.000000000 5.680000000 15.000000000
C 0.000000000 7.100000000 15.000000000
And then, I calculated the dielectric constants using the following ph.x
input:
phonons of gr
&inputph
tr2_ph=1.0d-14,
prefix='noiso',
epsil=.true.,
ldisp=.true.,
nq1=3,
nq2=3,
nq3=1,
amass(1)=12,
fildyn='noiso.dynG',
/
The result of calculation does not seem to be physically reasonable
(very large negative value):
Dielectric constant in cartesian axis
( -6123.274083490 0.000008052 -0.000001008 )
( 0.000008023 -6123.274129001 -0.000006809 )
( -0.000000922 -0.000006871 1.074380904 )
Could you give me a clue on which part of my input files might cause this?
Best,
Jay
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Lorenzo Paulatto - Paris
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