[Pw_forum] problem on phonon

[bhabya sahoo]

sir
i am using quantum espresso 4.3 version
i want to study the phase transition of yitrium nitride(yn)
first by doing scf calculation at 0k
i found a phase transition of B1 to B2 transition at 137 gpa for this i used
a 14*14*14 kpoints mesh and convergence achived for scf is 10^-9
i used GGA psudo potentials of pbe type avalable in code of espresso website
for y and n
but when i did the calculation for phonon dispersion
B2 stucter instablity (negative frequency ) in phonon spectrum continues
upto 500 gpa why ?
for this i used 4*4*4* mesh q points for dyanamical matrices i have incresed
further but nothing results

On Sat, May 28, 2011 at 9:36 PM, Stefano de Gironcoli <degironc at sissa.it>wrote:

>  Dear Bhabya Sahoo,
>
>    your very stingy use of punctuation make quite difficult (at least for
> me) to follow your request...
>    Take your time and try to make you clear providing more detailed
> information about your system, what is your aim, what is the problem, what
> are the tests you have done.
>    No one is going to spend more time in thinking about your problem than
> you do in explaining it.
>
>    Please provide your affiliation.
>
>    stefano
> -
>  Stefano de Gironcoli, SISSA and DEMOCRITOS
>
>
> On 05/28/2011 05:46 PM, bhabya sahoo wrote:
>
> i am using quantum espresso static calculation obeys thermdyanamic stability
> transition from B1 to B2 transition at 138 gpa  so ihave used 24 24 24
> kpoint grid  for phonon in B2 phase is calculated 4 4 4 grid by linear
> response calculation but why the instability (negative frequency in phonon )
> upto 450 gpa  i have used pbe psudo potentials also the lattice constant is
> exact for experimental data
>  can metastability have negative frequency so why this occures
> optimisation is ok
> other problem why free energy is not coming for unstable phase for qha code
> in quantum espresso
> replay immidiate if u have solution
> On Sat, May 28, 2011 at 5:38 PM, bhabya sahoo <bdslipun at gmail.com> <bdslipun at gmail.com> wrote:
>
>
>  sir
> i  used to calculate the phonon calculation on a 4*4*4 grid in cscl phase
> but the unstability persists upto 500 gpa
> but it is the thermodyanamic stability after 140 gpa  kpoints 24*24*24
> what may be reseon i donot know
> please help me
>
>
>
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