[Pw_forum] What's wrong in my calculation of epsilon of fcc Au?

[gbliu]

Dear all,

       I want to calculate dielectric function of carbon nanotube, but I 
have no experiences of calculating dielectric function before. Hence, I 
decide to start with a simple case of fcc gold, because there are 
referable results from wien2k (wien2k is capable of simple case such as 
fcc Au, but too resource consumptive to deal with large cells such as CNT).
       Because epsilon.x doesn't support USPP, the UPF file of Au is got 
by converting fhi file from abinit using fhi2upf.x. I calculate fcc Au 
with a=7.71bohr and 15x15x15 k-mesh using both pwscf(v4.3) and wien2k. 
First I check the band structures and find that the results from both 
softwares agree well (see attachments). But the results of epsilon from 
pwscf is not satisfactory as the results from wien2k:

       (1) Figures imag_eps and real_eps are the compares with only 
interband contribution. (note that for pwscf, I subtract the intraband 
contribution [-omega_p^2/omega^2] to get only interband contribution for 
compares). We can see the differences are notable. What's the reason? 
BTW, I use a homogeneous k-mesh also 15x15x15 generated by myself for 
epsilon calculation (see input file fccAu.nscf4eps.in).
------input for epsilon.x--------
&inputpp
      outdir='./work'
      prefix='fccAu'
      calculation='eps'
/
&energy_grid
      smeartype='gauss'
      intersmear=0.1d0
      intrasmear=0d0
      wmax=20d0
      wmin=0d0
      nw=600
      shift=0d0
/
--------------------------------

      (2) As for the intraband contribution, wien2k gives the plasma 
frequency 9.0075eV which agrees well with experimental value (ref: 
http://wave-scattering.com/drudefit.html), but the result from epsilon.x 
is puzzling.
Specifically, plasma frequency given in the output of epsilon.x is 
dependent on the input parameters:
<1>. intrasmear=0,   wmax=20d0  -->   The bulk xx plasmon frequency [eV] 
is:    14.877926200
<2>. intrasmear=0,   wmax=10d0  -->   The bulk xx plasmon frequency [eV] 
is:    7.625822195
<3>. intrasmear=0,   wmax=6d0   -->   The bulk xx plasmon frequency [eV] 
is:    3.156554381
<4>. intrasmear=0.1, wmax=20d0  -->   The bulk xx plasmon frequency [eV] 
is:    15.766403193
<5>. intrasmear=0.1, wmax=10d0  -->   The bulk xx plasmon frequency [eV] 
is:    9.322251787
<6>. intrasmear=0.1, wmax=20d0  -->   The bulk xx plasmon frequency [eV] 
is:    6.263713572
However, none of these values is the one got by fitting the results 
using Drude model [ 1 - omega_p^2/(omega^2+i*omega*gamma), gamma is the 
intrasmear here ]. By fitting the results using Drude model, I get 
omega_p=5.528769eV, for all the cases above (<1>~<6>). But this omega_p 
is far from the experimental value (~9 eV)! What's wrong? And what are 
the meanings of the values given in <1>~<6> ?

       How can I get more accurate results like the ones from wien2k?
       Please help me if you know something about this, thanks in advance!

Best regards!

                    Liu Guibin
                Postdoctoral Fellow,
                Department of Physics,
                The University of Hong Kong
                gbliu at hku.hk





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