[Pw_forum] Electron-phonon coupling

[Malgorzata Wierzbowska]

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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