[Pw_forum] Should I scrap the nonanalytic part of DM in electroded thin films?

[Stefano Baroni]

Dear Serge:

>
> I understand that in bulk calculations the dynamical
> matrices at small q are split into analytic and
> nonanalytic parts, with the latter accounting for the
> effects of macroscopic electric field. However, if
> we want to study phonons in a model representing an
> electroded thin film instead of a bulk crystal (i.e.,
> the electrodes are not explicitly included in such a
> calculation), we should assume that such an electric
> field will be screened out by the free charges and
> its influence onto ionic vibrations should not be
> considered.
>
> I am under an impression that to achieve this in pwscf
> I need to block the calls to nonanal(...) subroutine
> in dynmat and matdyn, while leaving everything else
> intact. Would this work out or could it possibly be not
> as trivial as I imagine it?

I am afraid it is indeed not as trivial as you imagine it ... I am no  
longer familiar with the guts of the code, but the only thing the code  
can possibly do is to make the phonons calculated at q=0 equal to the  
values that the phonon branches would have in the long-wavelength  
limit (and those depend in a non trivial way on the combination of  
wavevector/polarization, because of the long range character of the  
Coulomb interaction). I think that, in the presence of free carriers,  
the results of the code are in principle correct whenever the phonon  
wavelength is smaller that the screening length. At q=0 just setting  
the effective charges equal to zero (which is what you would  
presumably get by commenting out some appropriate routine calls)  
should do the job, but you would then get frequencies that are  
inconsistent with those calculated at finite q. To get fully  
consistent results, I think that screening from free carriers should  
be explicitly accounted for in the calculation of the dynamical  
matrix. Possible, but "not as trivial as one could imagine".

Cheers - stefano

---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
http://www.sissa.it/~baroni / [+39] 040 3787 406 (tel) -528 (fax) /  
stefanobaroni (skype)

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