[QE-users] negative phonon frequency when calculate lambda

[Lorenzo Paulatto]

Hello Chao,
do you mean, if you can compute lambda, or you have computed it already and
you are asking if it makes sense?
THe big problem with negative frequencies with this kind of system is that
you cannot really impose conservation of energy, but as in elph
conservation of energy is maybe not crucial. Anyway, the original lambda.x
code does not do any check on the frequencies, so you probably get NaN or
mathematical errors if you try to use it.

Lambda.f90 is a very simple code, I think you may try to edit it and see if
different recipes can give something reasonable (i.e. skip a mode when w2
is negative, use a complex omega=sqrt(w2) but take delta( |e-omega|)...),
sum the contribution with a weight exp(-IMAG(omega)/something),....

Of course, playing with the code can help thinking, but does not replace it.

cheers

On Mon, 15 Oct 2018 at 18:22, Chao Lei <leichao.forum at gmail.com> wrote:

> Dear users,
>
> I am calculating electron-phonon using ph.x, which got negative frequency.
> This is OK for me as it is expected, however, I have a question that how
> does ph.x treat the negative frequency when use them to calculate the
> electron-phonon coupling constant lambda ? Thanks for your reply
>  if you know the answer.
>
> Best
>
> Chao Lei
> Pos-doc
> The University of Texas at Austin
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-- 
Lorenzo Paulatto - Paris
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