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

[Katalin Gaal-Nagy]

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
>>
>>
>>
>>
>>
>>
>>
>> ---------------------------------
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>
>
>
>
>
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