[Pw_forum] ele phonon coupling at the gamma point

[Matteo Calandra]

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> Dear all,I am trying to do el-ph calculations only at the Gamma point for graphene (as I want to compare it to another system where the calculations are only feasible at the Gamma point). However the values I am getting for lambda for graphene  AT THE GAMMA POINT are :
> degauss   lambda      0.005   13.442600   0.015    2.303600   0.020    1.756000     0.025    1.436000      0.030    1.228800      0.035    1.085800      0.040    0.983000      0.045    0.906400      0.050    0.847800I am using a 90 x90 x1 grid (in scf) for graphene and the ph file has only one point nq1=0 nq2=0, nq3=0 (the Gamma point).
> It is already known that lambda for graphene is  ~0.02. (1) Do the above results mean that one cannot calculate el-ph coefficients unless on a grid of phonons and then are the calculations -at the Gamma point ONLY- useless and unexact?
> (2) or does this mean that the results did not converge due to a small k-mesh in the scf calculations (90x90x1) and I need to increase it to say 400x400x1?
> Any suggestions of any kind would also be ahighly appreciated.
> Thanks for your help
> Elie MUniversity of NottsNG7 2RDUK 		 	   		

Dear Elie,


    as it is well known, the electron-phonon coupling (lambda) and the 
phonon linewidth (gamma) for an optical mode diverge at zone center, at 
least for what concerns the equation implemented in QE.
So it is pointless to consider more k-points and smaller smearings.
The issue is of course different if you consider
the electron-phonon matrix element ( |g_{kn,k+qm}^{nu}|^2 ) that is an 
object well defined at any q in the BZ for an optical mode.

You can have more details on the behaviour of the lambda or of the 
phonon linewidth in PRB 71, 064501 (2005) .

All the best,

  M.


-- 
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Matteo Calandra, Directeur de Recherche (DR2)
Institut de Mineralogie et de Physique des Milieux Condenses de Paris
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