[Pw_forum] problem on phonon

[bhabya sahoo]

On Sat, May 28, 2011 at 10:52 PM, bhabya sahoo <bdslipun at gmail.com> wrote:

> 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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>
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