[Pw_forum] negative el-ph coupling constant lambda

[Eyvaz Isaev]

Dear Max,

One can see that \lambda you got is quite small, just
around zero. So, a reason might be numerical noise for
such a kind small \lambda. \gamma depends also on
production of two delta functions which couples two
electronic states at the Fermi Surface and it
converges slowly. I suggest you should perform tests
with respect to number of k-points (more fine that you
used), and higher cutoff energies.  

Bests,
Eyvaz.
P.S. Just today I have got \lambda for a metal, around
0.5, and everything went smoothly.

--- ³¯ ¤ÖµØ <chen_shao_hua197 at yahoo.com.tw> wrote:

> Dear Eyvaz and forum's members,
> 
> When I calculated the el-ph coupling constant
> \lambda
> of material.I find some negative value
> 
>   Electron-phonon coupling constant, lambda
> 
>  Broadening   0.0100 lambda       0.0008 dos_el  
> 0.3719
>  Broadening   0.0200 lambda       0.0012 dos_el  
> 0.3674
>  Broadening   0.0300 lambda       0.0021 dos_el  
> 0.3565
>  Broadening   0.0400 lambda      -0.0098 dos_el  
> 0.3821
>  Broadening   0.0500 lambda      -0.0303 dos_el  
> 0.5231
>  Broadening   0.0600 lambda      -0.0394 dos_el  
> 0.7545
>  Broadening   0.0700 lambda      -0.0408 dos_el  
> 1.0409
>  Broadening   0.0800 lambda      -0.0382 dos_el  
> 1.3767
>  Broadening   0.0900 lambda      -0.0333 dos_el  
> 1.7603
>  Broadening   0.1000 lambda      -0.0274 dos_el  
> 2.1824
> 
> Also find some negative value in gam.line
> e.g.
>   24     0.0000  0.0000  0.0000  -0.0001  0.0000
> -0.0001    *****    0.0000  0.0000
> 
> But no negative frequency found in matdyn.freq
> 
> I thought -\lambda comes from -\gamma (phonon
> linewidth), and -\gamma  comes from -\omega
> (frequency) by its definitions.
> 
> So I wonder why I got -\lambda and -\gamma (It
> should
> be >=0)
> even frequency \omega seems fine (no negative) ???
> 
> Should I just try check convergence of \gamma in
> dependent on cutoff energy and k-points sampling ?
> (from forum's "search archive", title " 3 questions
> about elph file of Calculation of electron-phonon
> interaction coefficients" suggestions )
> 
> Thanks in advance
> 
> max
> PHD candidate
> Physics department, National Taiwan
> University,Taiwan
> 
> 
>      
>
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-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
IFM, Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, eyvaz_isaev at yahoo.com


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