[Pw_forum] Electron-phonon coupling

[ma Yanming]

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 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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