[Pw_forum] Electron-phonon coupling code

[Sridhar Sadasivam]

Dear espresso users and developers,

I am trying to read through the electron-phonon coupling code and have a
few questions.

1. From phq_readin.f90
  SELECT CASE( trim( electron_phonon ) )
  CASE( 'simple' )
     elph=.true.
     elph_mat=.false.
     elph_simple=.true.
  CASE( 'Wannier' )
     elph=.true.
     elph_mat=.true.
     elph_simple=.false.
     auxdvscf=trim(fildvscf)
  CASE( 'interpolated' )
     elph=.true.
     elph_mat=.false.
     elph_simple=.false.
  CASE DEFAULT
     elph=.false.
     elph_mat=.false.
     elph_simple=.false.
  END SELECT

What does the case electron_phonon = 'Wannier' correspond to? I see from
phonon.f90 that the code would call ep_matrix_element_wannier.f90 if
elph_mat = .true. but this does not seem to be explained anywhere in the
documentation (
http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PH.html#idp74656
)? Is this similar to an external program EPW which performs Wannier
interpolation from coarse to fine k-point grids.

2. From phonon.f90

           IF ( elph_mat ) then
              call ep_matrix_element_wannier()
           ELSE
              CALL elphon()
           END IF

elph_mat = .false. for both electron_phonon = 'simple' and electron_phonon
= 'interpolated'. So I guess elphon.f90 is invoked in both simple and
interpolated cases. Also in elphon.f90, there is no mention of the variable
'elph_simple'. It looks to me that both simple and interpolated cases would
lead to the same code being run. Where exactly is there a difference
between these two cases?

Thanks so much in advance!
Sridhar
Graduate Student
Purdue University
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