[Pw_forum] Re: [Pw_forum] »Ø¸´£º Re: [Pw_forum] negtive values for TA when using q2r.x

[Eyvaz Isaev]

Hi Katalin,

I just suggested that an unstable mode near Gamma
point might be a result of incorrectly determined
dynamical matrix (and then unstable mode) at X point,
and consequently, force constants obtained by q2r.x
are also not correct.  I am not sure that you will be
able to get correct behavior of the phonon spectrum 
(obtained using matdyn.x) with wrong interatomic force
constants. Are not you agree with this?

The ways how to try avoiding an unstable mode at X
point were done in my previous mail. 

Bests,
Eyvaz.

--- Katalin Gaal-Nagy
<katalin.gaal-nagy at physik.uni-regensburg.de> wrote:

> Hi Eyvaz,
> 
> I am a little bit lost with your answer, what has
> some "real" instability 
> at (100) to do with some "numerical" instability
> close to Gamma?
> 
> Best, Katalin
> 
> 
> > Hi,
> >
> > If I understood correctly, you have got imaginary
> (but
> > not negative!, as you mentioned) frequences for
> X(100)
> > point using 4x4x4 q-mesh. If so, the use of 6x6x6
> mesh
> > which also has (100) point is hopeless.
> > Appearance of imaginary frequency at (100) point
> is a
> > usual case for unstable compounds within NaCl
> > structure.
> >
> > I suggest, it is somewhat strange to do FFT
> (q2r.x)
> > having an imaginary frequency and then expect a
> stable
> > structure. These imaginary frequences near Gamma
> point
> > you found using q2r.x,  are, presumably, connected
> to
> > X(100) point instability.
> >
> > So, I guess, the problem might be due to:
> > a) low cutoff energy
> > b) insufficent k-points mesh
> > c) quality of a pseudopotential
> > d) check carefully your input file
> >
> > Best regards,
> > Eyvaz.
> >
> >
> > --- li yan <liyanpcl at yahoo.com.cn> wrote:
> >
> >>       Dear katalin,
> >>
> >>       I want to find the reason of the phase
> >> transiton for AgF by calculating the phonon
> >> dispersion curves. My intent is finding the soft
> >> mode at high pressure.
> >>
> >>  >Which was the first grid, which you used to
> >> calculate the dispersion?
> >>
> >>       AgF is fcc strcture, c/a is 1, so the mesh
> was
> >> initially chosen as 4*4*4.
> >>
> >>   At 3GPa, after calculating the eight special
> >> points, i found only one of them (0,0,1) has a
> >> negative mode.   Then  i did the calculation  of
> the
> >> phonon disperion curves with q2r.x, the
> frequences
> >> of TA near the gama point  are negtive.  Near
> gama
> >> point, i just selected one q, whose TA is
> negtive,
> >> to calculate the phonon, but the value of the TA
> is
> >> positive. I thought  the q mesh is  small, so i
> >> chose 6*6*6 mesh.
> >>
> >>  > Does the 6x6x6 mesh include also the points
> you
> >> calculated for checking.
> >>
> >>       The 6*6*6 mesh dosen't include the above
> >> modes which has negtive values, but i cannot find
> >> other ways to avoid the unexpected negtive
> values.
> >>
> >>   thank you for your advice.
> >>     best regards
> >>      Li Yan
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> ---------------------------------
> >>
> ÎÞÏÞÈÝÁ¿ÑÅ»¢Ïà²á£¬Ô­Í¼µÈ´óÏÂÔØ£¬³¬¿ìËٶȣ¬¸Ï¿ìÇÀ×¢£¡
> >
> >
> >
> >
> >
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