[QE-users] Metal-substituted semiconductor: treat as metal or insulator for phonons

[Paolo Giannozzi]

The problem is that the true system is not metallic, but in a band picture
it is, even for a supercell of 1m side. Maybe one might compute phonons as
for a metal, then use dielectirc constant and effective charges of the
insulating host crystal. Not sure what one can use for the effective
charges of Fe, though.

Paolo

On Sun, Mar 4, 2018 at 8:39 AM, Pietro Delugas <pdelugas at sissa.it> wrote:

> Hello Chris
>
> If the system is metallic, no matter how bad a conductor it can be, in a
> finite time it will be able to screen any constant electric field. This  is
> as to say that the static dielectric constant is infinite.
>
> Regards Pietro
>
>
> Il 04 mar 2018 8:03 AM, Christoph Wolf <wolf.christoph at qns.science> ha
> scritto:
>
> Dear all,
>
> I have a fairly general question and I hope I can pick someone's brain:
>
> If an insulator or semiconductor is doped with a metal narrow bands
> determined by the crystal field emerge and often the fermi level lies
> within one of the bands, i.e. the "bands cut the fermi level", which is
> often called a characteristic of a conductor but in the bigger picture no
> electrons would be able to cross from the VB to the CB, i.e. the host
> system is an insulator.
>
> When attempting to calculate the phonons of a Mg7O8Fe supercell the
> dieletric constant (in the case of pure MgO eps~3.1) cannot be computed
>
>      Electric Fields Calculation
>      ik   1 ibnd   0 linter: root not converged  2.635E-07
> ....
> ..
>
>      End of electric fields calculation
>
>           Dielectric constant in cartesian axis
>
>           (******************       0.000244141      -0.001708984 )
>           (      -0.000244141******************       0.000244141 )
>           (      -0.000732422       0.000732422****************** )
>
> And I am wondering if that system should be treated as a metal instead
> (epsil    = .false.,) during the phonon run?
>
> --
> &inputph
>   prefix   = 'MgO',
>   epsil    = .true.,
>   alpha_mix(1) =0.4
>   alpha_mix(2) =0.4
>   fildyn   = 'MgO.dyn',
>   ldisp    = .true.
>   fildvscf = 'dvscf'
>   amass(1) = 24.30500
>   amass(2) = 15.99900
>   amass(3) = 55.84500
>   outdir='./',
>   nq1=6,
>   nq2=6,
>   nq3=6,
>   tr2_ph   =  1.0d-12,
>  /
>
> Any hint is welcome!
>
> Thanks in advance for your help and a nice Sunday everyone!
>
> Chris
> --
> Postdoctoral Researcher
> Center for Quantum Nanoscience, Institute for Basic Science
> Ewha Womans University, Seoul, South Korea
>
>
>
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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