[Pw_forum] Electron-phonon coupling

Fri Mar 28 20:07:48 CET 2003

Dear Ma Yanming,

Find some of my results and look to attached files with Eliashberg function
calculated for Nb with norm conserving PP (KB) and ultrasoft (US).
Experimental total lambda is 1.04 or 1.22 (from refs in S.Y. Savrasov
PRB, vol 54, p. 16487 (1996)), calculated by Savrasov 1.26.

As you can see from attached files KB PP behaves correctly
but US gives too big total lambda. The reason is because
Eliashberg function, in case of US, doesn't fall down to zero for small
frequencies. Small frequencies are for phonon vectors close to Gamma point.
However for first sight we calculate different quantities we still can
make common
conclusions. You integrate only over k, leaving q-dependence (phonon).
I integrate over both k and q points, but with frequncy resolution (I
see the relations
between q-points and frequencies.).

The answer for the question whether you can just throw points is NO! It
is not so easy,
but we will get there. What I suggest is:
If you could write a file with q-points you calculated and corresponding
frequencies
and corresponding calculated el-ph couplings. And if you could send me
refs. to experimental
values which you refer. Maybe we will see that one needs to work a bit
more on generation of PP.
Especially concentrating attention to get gamma point very close to zero.

I am satisfied with the KB PP attached here (format for old version of
PWscf). And I was still
working on getting US PP better. But now I don't have enough computer
power to continue
because I lost an account in my previous place and here I work on a
different project.
Maybe you can generate better pseudos. If you decide to do this for US
PP, there is a parameter
called "rinner" in Vanderbilt's input. Phonons at gamma point are very
sensitive on that.

Good luck!
Gosia

ma Yanming wrote:

> 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
>> >>
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>> >
>> >
>> >
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