[QE-users] Negative values showing in real part of dielectric function of Silicon

[Md. Jahid Hasan Sagor]

Hi QE Experts,

To find out the dielectric function (imaginary and real parts) of silicon,
I have run scf.in, nscf.in, epsilon.in (I have shared results in the
attachment). I have compared the imaginary part of the dielectric function
with one study and the results are in good agreement. However, the problem
is in the real part of the dielectric function. It contains  several
negative values and due to this absorption coefficient I found, has not
matched with the study.  I am very confused why the real part shows absurd
value, if the imaginary part matches with the study. Would you please share
your opinions?
Here is the scf.in:
 &control
  calculation = 'scf'
  restart_mode = 'from_scratch'
  prefix = 'ZG-si'
  pseudo_dir = './'
  outdir = './out'
/
&system
  ibrav = 0
  nat =   16
  ntyp =    1
  ecutwfc =  90.00
/
&electrons
  diagonalization = 'david'
  mixing_mode= 'plain'
  mixing_beta = 0.70
  conv_thr =   1D-06
/
ATOMIC_SPECIES
   Si   28.085 Si.pz-hgh.UPF
K_POINTS automatic
   8   8   8   0   0   0
CELL_PARAMETERS (angstrom)
      5.38154150      5.38154150      0.00000000
      0.00000000      5.38154150      5.38154150
      5.38154150      0.00000000      5.38154150
ATOMIC_POSITIONS (angstrom)
%%%%%%
nscf.in
&control
  calculation = 'nscf'
  restart_mode = 'from_scratch'
  prefix = 'ZG-si'
  pseudo_dir = './'
  outdir = './out'
/
&system
  ibrav = 0
  nat =   16
  ntyp =    1
  ecutwfc =  90.00
  nbnd = 56
  nosym = .true.
  noinv = .true.
/
&electrons
  diagonalization = 'david'
  mixing_mode= 'plain'
  mixing_beta = 0.70
  conv_thr =   1D-06
/
ATOMIC_SPECIES
   Si   28.085 Si.pz-hgh.UPF

CELL_PARAMETERS (angstrom)
      5.38154150      5.38154150      0.00000000
      0.00000000      5.38154150      5.38154150
      5.38154150      0.00000000      5.38154150
ATOMIC_POSITIONS (angstrom)
K_POINTS crystal
%%%%%%
Here is the epsilon.in:
&inputpp
outdir      = './out',
prefix      = 'ZG-si',
calculation = 'eps'
/
&energy_grid
smeartype   = 'gauss',
intersmear  = 0.03d0,
wmax        = 4.5d0,
wmin        = 0.2d0,
nw          = 600,
shift       = 0.0d0,
/
%%%%%%

Thanks

Best Regards
Md Jahid Hasan
PhD student
Mechanical Engineering
University of Maine
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