[Pw_forum] Electron-phonon coupling

[ma Yanming]

Dear Gosia

Sorry for replying you so late. I'd already calculated the E_P coupling for 
most of possible q vectors to try to analyze the results. So It takes time. 
I use Norm-Conserving PPs. As what you said, for some q vectors with small 
phonon frequencies, the calculated E_P coupling is extremely large. I do 
not know why?

I use 16 16 16 Monkshort-Pack K mesh to generate q points for BCC Nb.
At q point (0 -0.0625 0.1875) the E_P coupling is worst. For q points as 
the following, the results are also bad
(0 -0.0625 0.4375)
(0 -0.125 0.5)
(0 -0.0625 0.3125)
(0 -0.125 0.375)
(0 -0.125 0.25)
(0 -0.1875 0.3125)
(-0.0625 -0.1875 0.25)
(-0.0625 -0.1875 0.375)
(-0.125 -0.1875 0.3125)  

So Can I just pick out these unusual q vectors and then make average over 
the left q points to get the E_P Coupling lambda. Or ...?

>It would help to check from which
>region at frequency axis it comes.

I do not quite understand the above sentence.Can you give me more 
explanation?
Thanks in advance.

Best Wishes!

Yanming Ma PhD
Steacie Institute for Molecular Sciences,
National Research Councils of Canada.
100 Sussex
K1A 0R6





>From: Malgorzata Wierzbowska <wierzbom at tcd.ie>
>Reply-To: pw_forum at pwscf.org
>To: pw_forum at pwscf.org
>Subject: Re: [Pw_forum] Electron-phonon coupling
>Date: Sat, 22 Mar 2003 13:03:41 +0000
>
>
>Which version of the program and what pseudopotentials do you use?
>If this is ultrasoft pseudopotetial then do as follows:
>Edit the file phonon.F (or phonon.f90) in phlib (or PH).
>Go to the end and you see the place when elph=.true..
>Write "call dvanqq" as it is below and recompile the program.
>
>if (elph) then
>call dvanqq
>if (.not.trans) call elphon
>call elphsum
>endif
>
>If pseudo is normconserving and you have too big el-ph couplings
>(with good phonon spectra) probably it happens at phonon vectors
>which have small frequencies.
>What exactly are the numbers? It would help to check from which
>region at frequency axis it comes.
>Which q-points are particularly bad?
>
>Gosia
>
>
>
>ma Yanming wrote:
>
> > Dear Paolo,
> >
> > Thanks for your reply.I do choose q points using Monkhorst-Pack K
> > mesh. But I am still upset. I already increased the K mesh to 48 48 48
> > when I calculated the EP coupling. To test the performance in several
> > high symmetry q if (elph) then points, I still got the results for
> > phonon linewidth far away from the experimental data, despite I
> > obtained the good results for phonon. :-(
> > Any comments?
> >
> > Best Wishes!
> >
> > Yanming Ma
> >
> >
> >> > I have been puzzled by the calculation of the electron-phonon (EP)
> >> > coupling coefficient lambda using PWSCF
> >>
> >> me too...
> >>
> >> The calculation of the electron-phonon coupling coefficients
> >> is presently done using a simple but not-so-smart algorithm:
> >> for each phonon q one calculates \lambda(q), performing a
> >> (slowly convergent) sum over k of electronic states.
> >>
> >> > My question is how to choose the q points properly in the first BZ?
> >>
> >> choose a uniform grid (Monkhorst-Pack, for instance) and pick q-points
> >> in the irreducible BZ. Unless your \lambda(q) is strongly anisotropic
> >> (this is the case of MgB_2, for instance: most of the contribution to
> >> \lambda comes from a specific phonon branch), the convergence wrt
> >> the q-grid shouldn't be a big problem.
> >>
> >> > So My question is what is the criterion for the EP coupling
> >
> > convergency.
> >
> >>
> >> it is more or less the same criterion that is used to check for
> >> convergence in metals: you should try increasingly dense k-point
> >> grids and a range of gaussian broadenings for each grid. You should
> >> find a range of gaussian broadening and of k-point grids for which
> >> the value of \lambda(q) is reasonably constant. Unfortunately this
> >> usually happens with rather large grids.
> >>
> >> There is some ongoing work on a better algorithm for calculating
> >> electron-phonon coupling coefficients using tetrahedra.
> >>
> >> Paolo
> >> --
> >> Paolo Giannozzi e-mail: giannozz at nest.sns.it
> >> Scuola Normale Superiore Phone: +39/050509412
> >> Piazza dei Cavalieri 7 Fax: +39/050509417, 050563513
> >> I-56126 Pisa, Italy Office: Lab. NEST, Via della Faggiola 19
> >>
> >> _______________________________________________
> >> Pw_forum mailing list
> >> Pw_forum at pwscf.org
> >> http://www.democritos.it/mailman/listinfo/pw_forum
> >
> >
> >
> > _________________________________________________________________
> > 享用世界上最大的电子邮件系统― MSN Hotmail。 http://www.hotmail.com
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
>
>
>
>
>_______________________________________________
>Pw_forum mailing list
>Pw_forum at pwscf.org
>http://www.democritos.it/mailman/listinfo/pw_forum


_________________________________________________________________
享用世界上最大的电子邮件系统― MSN Hotmail。  http://www.hotmail.com