[Pw_forum] negative frequency

[Stefano de Gironcoli]

If a negative (i.e. imaginary ) frequency is present at a non-zero q 
point this means that the system is unstable w.r.t a displacement at 
that q vector and therefore the unit cell for the (static) equilibrium 
geometry should actually be larger than the one you have chosen and 
should such that the unstable q-vector folds into the Gamma point of 
the  super-cell.
This is indeed the case for bcc Zr where if I remember correctly the bcc 
phase is stable only at high temperature due to anharmonic effects [see 
for instance PRB 31 6775-6778 (1985)  and PRL 58, 1769-1772 (1987) by 
Chen, Fu, Ho, and Harmon]

Stefano de Gironcoli - SISSA and DEMOCRITOS
 


Prasenjit Ghosh wrote:
> 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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