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Large value of electron-phonon coupling

https://forum.epw-code.org/viewtopic.php?t=186

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Large value of electron-phonon coupling

Posted: Thu Jan 26, 2017 2:33 pm
by JuniorKanga
Dear all,

I am trying to get the Eliashberg spectral function and the coupling constante for 2D structure. But in my a2f_tr.01 file I can see a very large value of the coupling constante. I tried to increase my k-grid from 50x50x1 to 100x100x1 and the same for q-grid but still the same values. This is what I can see:

Code: Select all

Integrated el-ph coupling
  #          194.5500485 195.2777750 196.5610817 198.5526233 201.5994074 206.0691786 211.8316591 218.2837682 224.7550854 230.7554247
 Phonon smearing (meV)
  #            0.0500000   0.1000000   0.1500000   0.2000000   0.2500000   0.3000000   0.3500000   0.4000000   0.4500000   0.5000000
 Electron smearing (eV)   1.0000000
 Fermi window (eV)  15.0000000
 Summed el-ph coupling   51.1575211


And my electron linewidth also seems incorrect.

Code: Select all

# Electron lifetime (meV)
#      ik       ibnd                 E(ibnd)      Im(Sgima)(meV)
        1          1   -0.91989997903537E+01    0.65294648205306E+04
        1          2   -0.31309522111578E+01    0.37843400680886E+04
        1          3   -0.23417061301380E+00    0.39613869181926E+04
        1          4   -0.23191659844129E+00    0.34995480223217E+04
        1          5    0.26404872449958E+00    0.23669191624573E+04
        1          6    0.20678734962920E+01    0.44778817622652E+04
        1          7    0.32530752316346E+01    0.44025357777756E+04
        1          8    0.32557881421891E+01    0.52157752596377E+04
        2          1   -0.91985513448746E+01    0.65265415293029E+04
        2          2   -0.31312869248670E+01    0.37821546565077E+04

here is my epw.in input file.

Code: Select all

 
--
&inputepw
  prefix      = 'Sn'
  amass(1)    = 118.71 
  outdir      = './'

  iverbosity  = 0

  elph        = .true.
  epbwrite    = .true.
  epbread     = .false.

  etf_mem     = .true.  

  epwwrite    = .true.
! mp_mesh_k = .true.
  epwread     = .false.

  nbndsub     =  8
  nbndskip    =  0

  wannierize  = .true.
  num_iter    = 300
  iprint      = 2
  dis_win_max = 12
  dis_froz_max= 7
  proj(1)     = 'random'   
  system_2d   = .true.
 

  elecselfen  = .true.
  phonselfen  = .true.
  a2f         = .true.

  parallel_k  = .true.
  parallel_q  = .false.

  fsthick     = 15 ! eV 
  eptemp      = 300 ! K 
  degaussw    = 1.0 ! eV

  dvscf_dir   = '/home/lab3/Desktop/phonon/save'
   filukk      = './Sn.ukk' 

  nqf1        = 100
  nqf2        = 100
  nqf3        = 1

  wdata(1) = 'search_shells=180'
 !wdata(2) = 'kmesh_tol=0.00001'

  nkf1        = 100
  nkf2        = 100
  nkf3        = 1

  nk1         = 16 
  nk2         = 16
  nk3         = 1

  nq1         = 8  
  nq2         = 8
  nq3         = 1
 
 /
             34 cartesian
         0.000000000   0.000000000   0.000000000   0.029 
         0.000000000   0.144337567   0.000000000   0.029
         0.000000000   0.288675135   0.000000000   0.029
         0.000000000   0.433012702   0.000000000   0.029
         0.000000000  -0.577350269   0.000000000   0.029
         0.125000000   0.072168784   0.000000000   0.029
         0.125000000   0.216506351   0.000000000   0.029
         0.125000000   0.360843918   0.000000000   0.029
         0.125000000   0.505181486   0.000000000   0.029
         0.125000000  -0.505181486   0.000000000   0.029
         0.125000000  -0.360843918   0.000000000   0.029
         0.125000000  -0.216506351   0.000000000   0.029
         0.125000000  -0.072168784   0.000000000   0.029
         0.250000000   0.144337567   0.000000000   0.029
         0.250000000   0.288675135   0.000000000   0.029
         0.250000000   0.433012702   0.000000000   0.029
         0.250000000   0.577350269   0.000000000   0.029
         0.250000000  -0.433012702   0.000000000   0.029
         0.250000000  -0.288675135   0.000000000   0.029
         0.250000000  -0.144337567   0.000000000   0.029
         0.250000000   0.000000000   0.000000000   0.029
         0.375000000   0.216506351   0.000000000   0.029
         0.375000000   0.360843918   0.000000000   0.029
         0.375000000   0.505181486   0.000000000   0.029
         0.375000000   0.649519053   0.000000000   0.029
         0.375000000  -0.360843918   0.000000000   0.029
         0.375000000  -0.216506351   0.000000000   0.029
         0.375000000  -0.072168784   0.000000000   0.029
         0.375000000   0.072168784   0.000000000   0.029
        -0.500000000  -0.288675135   0.000000000   0.029
        -0.500000000  -0.144337567   0.000000000   0.029
        -0.500000000   0.000000000   0.000000000   0.029
        -0.500000000   0.144337567   0.000000000   0.029
        -0.500000000  -0.866025404   0.000000000   0.029


It would be very helpful for me if anyone can give any suggestion.

Thank you in advance
Best

Mohammed V university
Morocco

Re: Large value of electron-phonon coupling

Posted: Thu Jan 26, 2017 3:32 pm
by roxana
Dear Mohammed,

It seems that you are using very large values for the Fermi window (15 eV) and electron smearing ( 1 eV). Have you checked if the Wannier band structure describes well the original band structure in the energy range (Ef-15, Ef+15), where Ef is the Fermi level?

Best,
Roxana

Re: Large value of electron-phonon coupling

Posted: Thu Jan 26, 2017 5:32 pm
by JuniorKanga
Dear Roxana,

Thank you very much for your reply.

The Fermi level is at -3,20 eV and the structure is gapless ( its band structure is similar to the graphene )
I didn't check if the Wannier band structure describes well the original band structure, but I thought that with a very large Fermi surface thickness I could get good result.

So I will try to decrease this value and also the electron smearing to 0.1 eV, and I will let you know if it worked.

Best

Junior Kanga

Mohammed V university of Morocco

Re: Large value of electron-phonon coupling

Posted: Tue Jan 31, 2017 11:20 pm
by JuniorKanga
Dear Roxana,

I have first plotted the band structure in the energy range (Ef-1, Ef+1) with electron smearing ( 0.01 eV) and it describes well the original band structure. Then I used 350x350x1 kpoint grid and 100x100x1 qpoint grid but i still get very large value of coupling constant:

Code: Select all

--
&inputepw
  prefix      = 'Sn'
  amass(1)    = 118.71 
  outdir      = './'

  iverbosity  = 3

  elph        = .true.
  epbwrite    = .true.
  epbread     = .false.

  etf_mem     = .true.  

  epwwrite    = .true.
! mp_mesh_k = .true.
  epwread     = .false.

  nbndsub     =  8
  nbndskip    =  0

  wannierize  = .true.
  num_iter    = 300
  iprint      = 2
  dis_win_max = 12
  dis_froz_max= 7
  proj(1)     = 'random'   
  system_2d   = .true.
 

  elecselfen  = .true.
  phonselfen  = .true.
  a2f         = .true.

  parallel_k  = .true.
  parallel_q  = .false.

  fsthick     = 1 ! eV 
  eptemp      = 300 ! K 
  degaussw    = 0.01 ! eV

  dvscf_dir   = '/home/lab3/Desktop/bandIn/EPW/phonon/save'
   filukk      = './Sn.ukk' 

  nqf1        = 100
  nqf2        = 100
  nqf3        = 1

  wdata(1) = 'search_shells=180'
 !wdata(2) = 'kmesh_tol=0.00001'

  nkf1        = 350
  nkf2        = 350
  nkf3        = 1

  nk1         = 16 ! must be consistent with the number of nscf kpoint used
  nk2         = 16
  nk3         = 1

  nq1         = 8  ! must be consistent the with the number of q points
  nq2         = 8
  nq3         = 1
 
 /
             34 cartesian
         0.000000000   0.000000000   0.000000000   0.029 
         0.000000000   0.144337567   0.000000000   0.029
         0.000000000   0.288675135   0.000000000   0.029
         0.000000000   0.433012702   0.000000000   0.029
         0.000000000  -0.577350269   0.000000000   0.029
         0.125000000   0.072168784   0.000000000   0.029
         0.125000000   0.216506351   0.000000000   0.029
         0.125000000   0.360843918   0.000000000   0.029
         0.125000000   0.505181486   0.000000000   0.029
         0.125000000  -0.505181486   0.000000000   0.029
         0.125000000  -0.360843918   0.000000000   0.029
         0.125000000  -0.216506351   0.000000000   0.029
         0.125000000  -0.072168784   0.000000000   0.029
         0.250000000   0.144337567   0.000000000   0.029
         0.250000000   0.288675135   0.000000000   0.029
         0.250000000   0.433012702   0.000000000   0.029
         0.250000000   0.577350269   0.000000000   0.029
         0.250000000  -0.433012702   0.000000000   0.029
         0.250000000  -0.288675135   0.000000000   0.029
         0.250000000  -0.144337567   0.000000000   0.029
         0.250000000   0.000000000   0.000000000   0.029
         0.375000000   0.216506351   0.000000000   0.029
         0.375000000   0.360843918   0.000000000   0.029
         0.375000000   0.505181486   0.000000000   0.029
         0.375000000   0.649519053   0.000000000   0.029
         0.375000000  -0.360843918   0.000000000   0.029
         0.375000000  -0.216506351   0.000000000   0.029
         0.375000000  -0.072168784   0.000000000   0.029
         0.375000000   0.072168784   0.000000000   0.029
        -0.500000000  -0.288675135   0.000000000   0.029
        -0.500000000  -0.144337567   0.000000000   0.029
        -0.500000000   0.000000000   0.000000000   0.029
        -0.500000000   0.144337567   0.000000000   0.029
        -0.500000000  -0.866025404   0.000000000   0.029


And my result looks like

Code: Select all

 Integrated el-ph coupling
  #          122.6416807 123.2526047 124.3259630 125.9731256 128.4444735 132.1113343 137.1130150 143.0961915 149.4051532 155.4077703
 Phonon smearing (meV)
  #            0.0500000   0.1000000   0.1500000   0.2000000   0.2500000   0.3000000   0.3500000   0.4000000   0.4500000   0.5000000
 Electron smearing (eV)   0.0100000
 Fermi window (eV)   1.0000000
 Summed el-ph coupling  122.4424743


Is it any problem with my input file or something else?
Any suggestion would be very helpful
Thank you in advance

Best

Junior Kanga

Mohammed V university of Morocco

Re: Large value of electron-phonon coupling

Posted: Wed Feb 01, 2017 11:26 am
by sponce
Hello,

From your input, I would be most worried about the Wannierization.

Usually using random projection do not lead to very good MLWF.

What is your final spread?

You should print the Wannierized BS to check that its correct. You can do so by doing


wannierize = .true.
num_iter = 300
iprint = 2
dis_win_max = 12
dis_froz_max= 7
proj(1) = 'random'
wdata(1) = 'bands_plot = .true.'
wdata(2) = 'begin kpoint_path'
wdata(3) = 'L 0.50 0.00 0.00 G 0.00 0.00 0.00'
wdata(4) = 'G 0.00 0.00 0.00 X 0.50 0.50 0.00'
wdata(5) = 'end kpoint_path'
wdata(6) = 'bands_plot_format = gnuplot'
wdata(7) = 'guiding_centres = .true.'
wdata(8) = 'dis_num_iter = 300'
wdata(9) = 'num_print_cycles = 10'
wdata(10) = 'dis_mix_ratio = 1.0'

Off course change my L-G-X path to the one that match your 2D materials.

You will then get file that you can plot with gnuplot. Make sure that you get the same bandstructure as with an pw.x nscf band calculations.

Best,

Samuel

Re: Large value of electron-phonon coupling

Posted: Fri Feb 03, 2017 10:49 am
by JuniorKanga
Dear Samuel Ponc,

I can get reasonable bands structure exactly like the other articles with this input but my coupling constant is too large.

About my final Spread I can see:

Code: Select all


         Spreads (Ang^2)       Omega I      =    76.811422973
        ================       Omega D      =     6.457229503
                               Omega OD     =    46.785178669
    Final Spread (Ang^2)       Omega Total  =   130.053831144


This is my input file:

Code: Select all

--
&inputepw
  prefix      = 'Sn'
  amass(1)    = 118.71 
  outdir      = './'

  iverbosity  = 3

  elph        = .true.
  epbwrite    = .true.
  epbread     = .false.

  etf_mem     = .true.  

  epwwrite    = .true.
  epwread     = .false.

  nbndsub     =  8
  nbndskip    =  0

  wannierize  = .true.
  num_iter    = 300
  iprint      = 2
  dis_win_max = 12
  dis_froz_max= 7
  proj(1)     = 'random'   
 

  elecselfen  = .true.
  phonselfen  = .true.
  a2f         = .true.


  parallel_k  = .true.
  parallel_q  = .false.

  fsthick     = 1 ! eV 
  eptemp      = 300 ! K 
  degaussw    = 0.01 ! eV
  degaussq    = 0.05 ! meV

  dvscf_dir   = '/home/lab3/Desktop/bandIn/EPW/phonon/save'
   filukk      = './Sn.ukk' 

  nqf1        = 150
  nqf2        = 150
  nqf3        = 1

  
   wdata(1) = 'bands_plot = .true.'
   wdata(2) = 'begin kpoint_path'
   wdata(3) = 'G 0.00 0.00 0.00 K 0.3333 0.3333 0.00'
   wdata(4) = 'K 0.3333 0.3333 0.00 M 0.00 0.50 0.00'
   wdata(5) = 'M 0.00 0.50 0.00 G 0.00 0.00 0.00'
   wdata(6) = 'end kpoint_path'
   wdata(7) = 'bands_plot_format = gnuplot'
   wdata(8) = 'guiding_centres = .true.'
   wdata(9) = 'dis_num_iter = 300'
   wdata(10) = 'num_print_cycles = 10'
   wdata(11) = 'dis_mix_ratio = 1.0'
   wdata(12) = 'search_shells=180'


  nkf1        = 150
  nkf2        = 150
  nkf3        = 1

  nk1         = 16 ! must be consistent with the number of nscf kpoint used
  nk2         = 16
  nk3         = 1

  nq1         = 8  ! must be consistent the with the number of q points
  nq2         = 8
  nq3         = 1
 
 /
             34 cartesian
         0.000000000   0.000000000   0.000000000   0.029 
         0.000000000   0.144337567   0.000000000   0.029
         0.000000000   0.288675135   0.000000000   0.029
         0.000000000   0.433012702   0.000000000   0.029
         0.000000000  -0.577350269   0.000000000   0.029
         0.125000000   0.072168784   0.000000000   0.029
         0.125000000   0.216506351   0.000000000   0.029
         0.125000000   0.360843918   0.000000000   0.029
         0.125000000   0.505181486   0.000000000   0.029
         0.125000000  -0.505181486   0.000000000   0.029
         0.125000000  -0.360843918   0.000000000   0.029
         0.125000000  -0.216506351   0.000000000   0.029
         0.125000000  -0.072168784   0.000000000   0.029
         0.250000000   0.144337567   0.000000000   0.029
         0.250000000   0.288675135   0.000000000   0.029
         0.250000000   0.433012702   0.000000000   0.029
         0.250000000   0.577350269   0.000000000   0.029
         0.250000000  -0.433012702   0.000000000   0.029
         0.250000000  -0.288675135   0.000000000   0.029
         0.250000000  -0.144337567   0.000000000   0.029
         0.250000000   0.000000000   0.000000000   0.029
         0.375000000   0.216506351   0.000000000   0.029
         0.375000000   0.360843918   0.000000000   0.029
         0.375000000   0.505181486   0.000000000   0.029
         0.375000000   0.649519053   0.000000000   0.029
         0.375000000  -0.360843918   0.000000000   0.029
         0.375000000  -0.216506351   0.000000000   0.029
         0.375000000  -0.072168784   0.000000000   0.029
         0.375000000   0.072168784   0.000000000   0.029
        -0.500000000  -0.288675135   0.000000000   0.029
        -0.500000000  -0.144337567   0.000000000   0.029
        -0.500000000   0.000000000   0.000000000   0.029
        -0.500000000   0.144337567   0.000000000   0.029
        -0.500000000  -0.866025404   0.000000000   0.029


Is the problem about the projections proj(1) = 'random' ? Because I have tried proj(1) = 'Sn:sp3' instead of sp2 ( because my structure is not planar ) but it didn't work. I would like to know if there is anything wrong in input.

Thank you in advance

Best

Re: Large value of electron-phonon coupling

Posted: Fri Feb 03, 2017 11:05 am
by sponce
Hello,

This depends on how many atoms you have in your unit cell.

What is the final spread on each atoms? Ideally it should be below 4.

If you have few atoms, a total spread of 130 might be very large (i.e. not so good).

Did you plot the bandstructure obtained with Wannier with the one obtained with bands.x for QE ?
They should overlay perfectly.

You should indeed try something like proj(1) = 'Sn:sp3'. If that does not work maybe some other parameters in Wannier are not good.
Try to play with disantanglements etc.

You can also try proj(1) = 'Sn:p' and proj(2) = 'Sn:s'

Best,

Samuel

Re: Large value of electron-phonon coupling

Posted: Sat Feb 04, 2017 1:55 pm
by JuniorKanga
Dear Samuel

Thank you very much for your quick reply and suggestions. I didn't checked if bands structure overlay perfectly, I just focused on the shape.
I have tried to use proj(1)= 'Sn:p' and proj(2)= 'Sn:s' and the code doesn't complain (thank you very much).
But the final spread is still to large. I will try to play with disantanglements, the fsthick and the k-points.
I will let you know if something works.

Best

Junior Kanga
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