[QE-users] negative phonon frequency when calculate lambda

[Chao Lei]

Hello, Lorenzo,

>From the user guide of phonon, here is the introduction of matdyn.x:

PH/matdyn.x: produces phonon frequencies at a generic wave vector using the
IFC file
calculated by q2r.x; may also calculate phonon DOS, the electron-phonon
coefficient λ,
the function α2 F (ω). And I do have a output file of lambda, is it because
the latest version of matdyn.x that can do the calculation of lambda ?

This is just referred fromquantum-espresso/PHonon/examples/example03/
*run_example*

Best

Chao Lei


On Mon, 29 Oct 2018 at 03:52, Lorenzo Paulatto <paulatz at gmail.com> wrote:

> Matdyn just interpolates the 2-body force constants to get the dynamical
> matrix at an arbitrary q point and diagonalizes it. It does not care if
> the frequencies are positive or negative, it also does not compute any
> electron-phonon coupling.
>
> kind regards
>
> On 23/10/2018 23:21, Chao Lei wrote:
> > Hello, Lorenzo,
> >
> > Thanks for your reply, I am using matdyn.x and it already got the lambda
> > vs broadening without NaN or mathematical errors, the negative frequency
> > makes sense for me. So in matdyn.x the code does not do any check on the
> > frequencies too ? or it just neglected the negative frequency when doing
> > the sum ?
> >
> > Best wishes
> >
> > Chao Lei
> > Pos-doc
> > The University of Texas at Austin
> >
> > On Tue, 16 Oct 2018 at 02:43, Lorenzo Paulatto <paulatz at gmail.com
> > <mailto:paulatz at gmail.com>> wrote:
> >
> >     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
> >     <mailto: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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> >         users at lists.quantum-espresso.org
> >         <mailto:users at lists.quantum-espresso.org>
> >         https://lists.quantum-espresso.org/mailman/listinfo/users
> >
> >
> >
> >     --
> >     Lorenzo Paulatto - Paris
> >     _______________________________________________
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> >     users at lists.quantum-espresso.org
> >     <mailto:users at lists.quantum-espresso.org>
> >     https://lists.quantum-espresso.org/mailman/listinfo/users
> >
> >
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> >
>
> --
> Lorenzo Paulatto - Paris
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