[Pw_forum] Re: Different Gamma factors for Q-points in the same star

[Andrea Marini]

On Mon, 29 May 2006, Eyvaz Isaev wrote:

> I am quite agree with your comment, but I suggested
> (and hopefully it is) that the symmetrization is done
> correctly in the code.  As you know the gamma
> approximately is a sum of production of two delta
> functions, connecting two parts of the Fermi surface,
> and is believed to converge slowly wrt number of
> k-sampling. That is why I suggested the use of  more
> k-points.

Dear Eyvaz & Stefano,

 	I do agree as well about what Stefano commented. And if we agree 
this means that using also a small set of k-points symmetry relations 
between the different gamma factors should be respected.

So, unless I am doing something wrong in my calculations (perfectly 
possible) there could be something wrong in the code.

Now, after studying the symmetrization of the dynamical matrix 
(symdynph_gq.f90) I understood (also thanks to Paolo comments) that it 
simply expands the sum on the k-points entering the gamma factor 
expression  to the whole BZ starting from the k-points reduced using the 
small group of q.

To my understanding, however, this  symmetrization of the electron-phonon 
matrix elements is not necessary as the electron-phonon matrix elements 
are more symmetric than the dynamical matrix (being a dot product).

Strictly speaking if I remove the call to

call symdynph_gq (xq, phi, s, invs, rtau, irt, irgq, nsymq, nat, &
                          irotmq, minus_q)

in symdyn_munu.f90 ONLY when this is called from elphon.f90 (in order to 
leave unchanged the symmetrization of the dynamical matrix) than I get the 
correct result for 2 the q-points of the same star.

Andrea

**************************************************************************
      q = (    0.250000000   0.750000000  -0.250000000 )

      omega( 1) =       2.619114 [THz] =      87.364805 [cm-1]
      omega( 2) =       3.738462 [THz] =     124.702500 [cm-1]
      omega( 3) =       4.666846 [THz] =     155.670255 [cm-1]

       Gaussian Broadening:   0.010 Ry, ngauss= -99
      DOS =  6.804424 states/spin/Ry/Unit Cell at Ef=  7.742912 eV
      double delta at Ef = 58.673351
      lambda( 1)=  2.1676   gamma=   96.62 GHz
      lambda( 2)=  1.0954   gamma=   99.47 GHz
      lambda( 3)=  0.1137   gamma=   16.09 GHz

**************************************************************************
      q = (    0.250000000  -0.250000000   0.750000000 )

      omega( 1) =       2.619204 [THz] =      87.367818 [cm-1]
      omega( 2) =       3.738232 [THz] =     124.694832 [cm-1]
      omega( 3) =       4.666853 [THz] =     155.670507 [cm-1]

      Gaussian Broadening:   0.010 Ry, ngauss= -99
      DOS =  6.804424 states/spin/Ry/Unit Cell at Ef=  7.742912 eV
      double delta at Ef = 58.673351
      lambda( 1)=  2.1675   gamma=   96.62 GHz
      lambda( 2)=  1.0955   gamma=   99.47 GHz
      lambda( 3)=  0.1137   gamma=   16.09 GHz

--------------------------------------------------------------------------
                                Andrea MARINI

        Physics Department, University of Rome "Tor Vergata" (Italy)
               - phone: +39-0672594894 - fax: +39-062023507 -

-- andrea.marini at roma2.infn.it - http://www.fisica.uniroma2.it/~marini/ --