EPW Forum

Posted: **Sun Aug 09, 2020 7:22 pm**

Dear Zishen:

I note that another thing different from epw and qe:
The prints from EPW/src/ephwann_shuffle.f90 doesn't mention the square of the electron-phonon matrix, and in EPW/src/selfen.f90:

In the comments of the subroutine you mentioned, that expression is for the case that the delta approximation is not used and in that expression, the part including the electron-phonon (e-ph) vertex is just omitted [Please pay your attention to the "sans"]; the e-ph vertex is correctly included in the actual calculation in EPW.

Since you are using the delta approximation (you set delta_approx= .true. in your EPW input) [See J. Noffsinger et al, Comput. Phys. Comm. 181, 2140 (2010)], the expressions used in the subroutine of selfen_phon_q in EPW/src and that of elphsum in PHonon/PH are the same. Both of them consider the spin degeneracy.
As I said, the only difference is that in EPW, the thing printed has an additional factor of 2 [The printing of phonon linewidth is done in EPW/src/ephwann_shuffle.f90] according to the expression of \Gamma = - 2 * Im \Pi^R .

And I compare curves between phonon dispersion and linewidth, the epw linewidth equal to zero points is exactly the imaginary phonon dispersion points. I think this is the reason for difference and it may affect other electron-phonon coupling calculations (electron-phonon matrix? electron-phonon constant?).

This is related to the different thing.
In EPW, when the phonon frequency is less than eps_acustic (See https://docs.epw-code.org/doc/Inputs.html#eps-acustic), some properties are ignored (set to zero). In your case, this leads to the zero phonon linewidth for the q values where the phonon frequency is less than eps_acustic.

(Excerpt from the subroutine of selfen_phon_q in EPW/src/selfen.f90)

Code: Select all

      wq(:) = zero
      DO imode = 1, nmodes
        wq(imode) = wf(imode, iq)
        IF (wq(imode) > eps_acustic) THEN
          g2_tmp(imode) = one
          inv_wq(imode) = one / (two * wq(imode))
        ELSE
          g2_tmp(imode) = zero
          inv_wq(imode) = zero
        ENDIF
      ENDDO

Sincerely,

H. Lee

Posted: **Fri Aug 21, 2020 4:06 am**

Dear H. Lee,

Thank you!
The tag eps_acustic works for phonon linewidth. After considering the factor of 2: the linewidths between qe and epw are very similar except for small difference:

: epw&qe linewidth.png (57.95 KiB) Viewed 2739 times

I also calculated electron-phonon coupling matrix g (lowest phonon - the band which cross fermi surface) and electron-phonon coupling constant lamda, and I find the imaginary frequency also induce problem in them (I have added eps_acustic):

: matrix & lamda.png (17.3 KiB) Viewed 2739 times

I was wondering if there is a relevant tag to solve this problem? And for the case of lamda, it is said it typically range between 0~2. The unreasonable large lamda may also because: lamda=phonon-linewidth/(pi*hbar*N(0)*w^2)?
N(0): density of fermi surface
w: phonon frequency.
As our structure has imaginary phonon frequency, the phonon curve will have very small w, this will induce large lamda as observed? I am not sure if I mkae mistake here. And in previous paper (prb 99, 161119 (2019)), they used large degauss to overcome the problem of imaginary frequency, then obtaining lamda.

Sincerely,
Zishen

EPW Forum

EPW phonon linewidth inconsistent with QE results

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