[Pw_forum] negative frequency

[Prasenjit Ghosh]

Dear Partha Sarathi,

I am not an expert on phonon calculation and I am also not familiar
with your system; however, an imaginary (negative) phonon mode at a
non zero q-vector usually indicates that the cell parameters (crystal
lattice vectors) which your determined, does not correspond to the
equilibrium one. Have you checked how the stresses are with your
equilibrium geometry?

Also at gamma point, are you getting 3 zero modes and rest non-zero modes?

With regards,
Prasenjit

On 21 May 2010 11:41, partha sarathi ghosh <parthasarathi13 at gmail.com> wrote:
> Dear all,
>                            Thank you xirainbow and Michael J. Mehlfor for
> your reply & suggestion .
>  I was trying to calculate longitudinal phonon dispersion of Zr along [111]
> direction.
> I wanted to validate my calculation with experimental results for bcc-Zr
> (not hcp) given in the paper :Phy.Rev.B,vol-29,page 1575-1587(1984).
> But I am getting negative frequencies near q=2/3(1 1 1) and the over all
> appearance of the curve is different from the experimental one.  I optimized
> the system w.r.t  ecutwf, ecutrho , k-point sampling , volume ,degauss,
> conv_thr. I have also tried with different q-point sampling in ph.in file
> (like 4 4 4, 8 8 8) with tr2_ph=1.0d-12.
> My questions are :
> 1. Is DFTP suitable for studying particular phonon mode like in Zr ??
> 2. If yes then how to make my calculations efficient ??
> 3. If not then any suggestion of other methods ??
>  Any one can please help me.
> Thanks in advance.
> P. S. Ghosh
> B.A.R.C.
> INDIA
>
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>



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