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

[Andrea Marini]

Dear users,

 	I am a newbie in using pw/ph to get electron-phonon coupling 
matrix elements (ME).

Now I have the following problem. To understand the ME symmetry properties 
I have done two calculations of the Eliashberg function for Al using a 
Monkhorst-Pack set of Q-points in the whole BZ and only in the irreducible 
wedge and I have found tiny (but not negligible) differences in the two 
calculations.

I have traced back the problem to different Gamma factors (calculated  in 
elphonon.f90) for two Q-points in the same star. This confuses me as I was 
expecting possible differences only  for degenerate modes.

Am I wrong ? Am I doing sone stupid mistake ?

THANKS!

Andrea

  **************************************************************************

      q = (    0.250000000   0.750000000  -0.250000000 )

      omega( 1) =       2.619122 [THz] =      87.365086 [cm-1]
      omega( 2) =       3.738457 [THz] =     124.702317 [cm-1]
      omega( 3) =       4.666853 [THz] =     155.670490 [cm-1]

      Gaussian Broadening:   0.100 Ry, ngauss=   1
      DOS =  4.376674 states/spin/Ry/Unit Cell at Ef=  8.063192 eV
      double delta at Ef = 33.847218
      lambda( 1)=  1.2622   gamma=   36.19 GHz
      lambda( 2)=  0.9349   gamma=   54.61 GHz
      lambda( 3)=  0.3583   gamma=   32.61 GHz

  **************************************************************************

      q = (    0.250000000  -0.250000000   0.750000000 )

      omega( 1) =       2.619570 [THz] =      87.380025 [cm-1]
      omega( 2) =       3.738907 [THz] =     124.717352 [cm-1]
      omega( 3) =       4.666851 [THz] =     155.670416 [cm-1]

      Gaussian Broadening:   0.100 Ry, ngauss=   1
      DOS =  4.376674 states/spin/Ry/Unit Cell at Ef=  8.063192 eV
      double delta at Ef = 33.847218
      lambda( 1)=  1.7620   gamma=   50.53 GHz
      lambda( 2)=  0.9916   gamma=   57.94 GHz
      lambda( 3)=  0.1641   gamma=   14.93 GHz

To obtain these results I have used

pw.x< Al.scf.in
pw.x< Al-1.nscf.in
ph.x< Al-1.ph_elph.in

for q =  0.250000000   0.750000000  -0.250000000

and

pw.x< Al.scf.in
pw.x< Al-2.nscf.in
ph.x< Al-2.ph_elph.in

for q =  0.250000000  -0.250000000   0.750000000

  = Al.scf.in =============================================================
  &control
     calculation = 'scf',prefix='Al',
     pseudo_dir = '/home/marini/Sources/espresso/espresso-3.0/pseudo/',
     wf_collect=.true.,
  /
  &system
     ibrav=  2, celldm(1) =7.50, nat=  1, ntyp= 1,ecutwfc = 16, nbnd = 5,
     occupations='smearing', smearing='mp', degauss=0.022
  /
  &electrons
     diagonalization='david'
     conv_thr = 1.0d-10
  /
ATOMIC_SPECIES
  Al  26.98  Al.vbc.UPF
ATOMIC_POSITIONS
  Al 0.00 0.00 0.00
K_POINTS {automatic}
   4 4 4 0 0 0

  = Al-1.nscf.in ==========================================================
  &control
     calculation='phonon',prefix  = 'Al'
     pseudo_dir = '/home/marini/Sources/espresso/espresso-3.0/pseudo/',
  /
  &system
     ibrav=  2, celldm(1) =7.50, nat=  1, ntyp= 1,ecutwfc = 16, nbnd = 5,
     occupations='smearing', smearing='mp', degauss=0.022
  /
  &electrons
   diago_thr_init =  1.0d-6
  /
  &phonon
     xqq(1)= 0.25  , xqq(2)= 0.75 , xqq(3)= -0.25
  /
ATOMIC_SPECIES
  Al  26.98  Al.vbc.UPF
ATOMIC_POSITIONS
  Al 0.00 0.00 0.00
K_POINTS {automatic}
   4 4 4 0 0 0

  = Al-1.ph_elph.in =======================================================

phonons of Al
  &inputph
   tr2_ph=1.0d-14,
   prefix='Al',
   amass(1)=26.98,
   fildyn='Al.dyn',
   fildvscf='Al.dvscf',
   filelph='Al-1.elph',
   iverbosity = 1,
   elph=.true.,
   trans=.true.,
  /
  0.25  0.75 -0.25

  = Al-2.nscf.in ==========================================================

  &control
     calculation='phonon',prefix  = 'Al'
     pseudo_dir = '/home/marini/Sources/espresso/espresso-3.0/pseudo/',
  /
  &system
     ibrav=  2, celldm(1) =7.50, nat=  1, ntyp= 1,ecutwfc = 16, nbnd = 5,
     occupations='smearing', smearing='mp', degauss=0.022
  /
  &electrons
   diago_thr_init =  1.0d-6
  /
  &phonon
     xqq(1)= 0.25  , xqq(2)= -0.25 , xqq(3)= 0.75
  /
ATOMIC_SPECIES
  Al  26.98  Al.vbc.UPF
ATOMIC_POSITIONS
  Al 0.00 0.00 0.00
K_POINTS {automatic}
   4 4 4 0 0 0

  = Al-2.ph_elph.in =======================================================

phonons of Al
  &inputph
   tr2_ph=1.0d-14,
   prefix='Al',
   amass(1)=26.98,
   fildyn='Al.dyn',
   fildvscf='Al.dvscf',
   filelph='Al-2.elph',
   iverbosity = 1,
   elph=.true.,
   trans=.true.,
  /
  0.25  -0.25 0.75


--------------------------------------------------------------------------
                                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/ --