[Pw_forum] Electron-phonon coupling

[Eyvaz Isaev]

Dear Malgorzata,

Thanks a lot for your comment. It works!!!

Right after editing phonon.f90 one has also to comment
173-174 lines in phq_readin.F90 before recompiling
phonon.

Regards 
Eyvaz.
--- Malgorzata Wierzbowska <wierzbom at tcd.ie> wrote:
> 
> 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
> >>
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> >
> >
> >
> >
>
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