[QE-users] dielectric constant calculation

[Lorenzo Paulatto]

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