[Pw_forum] different results for electron-phonon calculation for the same q-point

[Paolo Giannozzi]

On Wed, 2014-06-25 at 10:42 +0200, Florian Rittweger wrote:

> I calculate the electron-phonon coupling strength for bulk aluminum with 
> QE V.5.1, using two grids of q-points - 222 and 444. Looking at a 
> specific q-point which is contained in both grids, namely q = (0.0, 
> -1.0, 0.0), the results for the electron-phonon coefficient lambda and 
> the linewidth gamma differs. Additionally i did the electron-phonon 
> calculation for the single q-point with ldisp=.false., yielding 
> different results as well.

the results for degenerate modes depend upon which linear combination
of phonon modes results from the diagonalization. This is basically
unpredictable. It is more puzzling that you get different results 
for the non-degenerate mode. The safer results should be those 
produced by a single phonon calculation and electron_phonon='simple'.

Reminder: the calculation of electron-phonon coefficients is a tricky
business. While I have no evidence that it doesn't work, I have no
evidence either of the opposite

P.

> electron-phonon input:
> 
> 222-Q-POINT-GRID
>   &inputph
>    ...
>    ldisp=.true.
>    nq1=2, nq2=2, nq3=2
>   /
> 
> 444-Q-POINT-GRID
>   &inputph
>    ...
>    ldisp=.true.
>    nq1=4, nq2=4, nq3=4
>   /
> 
> SINGLE Q-POINT
>   &inputph
>    ...
>    ldisp=.false.
>   /
> 0.000 -1.000 0.000
> 
> The flags electron_phonon='interpolated', trans=.true., 
> el_ph_sigma=0.005 and el_ph_nsigma=1 are used in all calculations.
> 
> electron-phonon output:
> 
> 222-Q-POINT-GRID
> 
>   Calculation of q =    0.0000000  -1.0000000   0.0000000
>   ...
>   **************************************************************************
>       freq (    1) =       4.913252 [THz] =     163.888446 [cm-1]
>       freq (    2) =       4.913252 [THz] =     163.888446 [cm-1]
>       freq (    3) =      10.206764 [THz] =     340.461002 [cm-1]
>   **************************************************************************
>       Mode symmetry, D_4h(4/mmm) point group:
>       freq (  1 -  2) =        163.9  [cm-1]   --> E_u  X_5' M_5'
>       freq (  3 -  3) =        340.5  [cm-1]   --> A_2u X_4' M_4'
>       electron-phonon interaction  ...
>       Gaussian Broadening:   0.005 Ry, ngauss=   0
>       DOS =  5.347024 states/spin/Ry/Unit Cell at Ef=  8.438471 eV
>       lambda(    1)=  1.4445   gamma=  178.06 GHz
>       lambda(    2)=  1.1812   gamma=  145.60 GHz
>       lambda(    3)=  0.3285   gamma=  174.76 GHz
> 
> 444-Q-POINT-GRID
> 
>   Calculation of q =    0.0000000  -1.0000000   0.0000000
>   ...
>   **************************************************************************
>       freq (    1) =       4.913092 [THz] =     163.883106 [cm-1]
>       freq (    2) =       4.913092 [THz] =     163.883106 [cm-1]
>       freq (    3) =      10.205145 [THz] =     340.406980 [cm-1]
>   **************************************************************************
>       Mode symmetry, D_4h(4/mmm) point group:
>       freq (  1 -  2) =        163.9  [cm-1]   --> E_u  X_5' M_5'
>       freq (  3 -  3) =        340.4  [cm-1]   --> A_2u X_4' M_4'
>       electron-phonon interaction  ...
>       Gaussian Broadening:   0.005 Ry, ngauss=   0
>       DOS =  5.347024 states/spin/Ry/Unit Cell at Ef=  8.438471 eV
>       lambda(    1)=  1.0271   gamma=  126.59 GHz
>       lambda(    2)=  1.5098   gamma=  186.09 GHz
>       lambda(    3)=  0.4284   gamma=  227.80 GHz
> 
> SINGLE Q-POINT
> 
>   Calculation of q =    0.0000000  -1.0000000   0.0000000
>   ...
>   **************************************************************************
>       freq (    1) =       4.913143 [THz] =     163.884808 [cm-1]
>       freq (    2) =       4.913143 [THz] =     163.884808 [cm-1]
>       freq (    3) =      10.206794 [THz] =     340.462013 [cm-1]
>   **************************************************************************
>       Mode symmetry, D_4h(4/mmm) point group:
>       freq (  1 -  2) =        163.9  [cm-1]   --> E_u  X_5' M_5'
>       freq (  3 -  3) =        340.5  [cm-1]   --> A_2u X_4' M_4'
>       electron-phonon interaction  ...
>       Gaussian Broadening:   0.005 Ry, ngauss=   0
>       DOS =  5.347024 states/spin/Ry/Unit Cell at Ef=  8.438471 eV
>       lambda(    1)=  1.2835   gamma=  158.19 GHz
>       lambda(    2)=  1.2835   gamma=  158.19 GHz
>       lambda(    3)=  0.3946   gamma=  209.89 GHz
> 
> 
> The dynamical matrices are equal in all three cases but the eigenvectors 
> (and therefor the values for lambda and gamma?) differ with respect to 
> direction and/or magnitude.
> As far as i thought all three calculations should give the same results. 
> Is there an explanation why this shoudn't be true?
> 
> Any hints, discussion or explanation would be helpful.
> 
> With best regards,
> Florian
> 
> 
> -------------------------------------------------------------
> Florian Rittweger, PhD student
> Max Planck Institute of Microstructure Physics
> Von-Seckendorff-Platz 1, Room 1.07
> D-06120 Halle/Saale, Germany
> Tel.: ++49 345 5525462
> -------------------------------------------------------------
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum

-- 
 Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
 Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
 Phone +39-0432-558216, fax +39-0432-558222