[Pw_forum] Electron-phonon coupling

[Paolo Giannozzi]

On Wednesday 19 March 2003 16:43, ma Yanming wrote:

> 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
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