Re: [Phonopy-users] [help] Query for translation imaginary modes taken into account by phonopy

[Atsushi T. <atz...@gm...>]

Hi Asif,

It's not only about theory. Otherwise we don't need to use computer
simulation. We always have technical issues.

Togo

On Fri, Feb 21, 2020 at 4:19 AM BHATTI Asif <Asi...@ls...> wrote:
>
> Hi,
>
> I tried both approaches I did not see any difference.  Well I will use --fc-symmetry option.
>
> Just to tell you I don't think you need a large supercell for DFPT calculation only if you use frozen phonon calculation then you need to consider a big cell because of the perturbative field effect resulting from the displacement of the atoms within a small cell. The mathematical formalism of DFPT barred us in using the large cell. I think we should read the original paper regarding the assumption being laid.
>
> However, these translation imaginary could be removed I think if I use ADDGRID or increase the NGKX grid to accurate.
>
> Regards,
> Asif
> ________________________________
> From: Alex Kent [ake...@gm...]
> Sent: Thursday, February 20, 2020 2:36 PM
> To: pho...@li...
> Subject: Re: [Phonopy-users] [help] Query for translation imaginary modes taken into account by phonopy
>
> Dear Asif,
>
> As prof. Togo should you should use finite displacement method. Also make sure to use large enough super cell.
>
> Also you should use --fc-symmetry option when plotting the phonon dispersion. This may improve the translation invariance, and may improve the phonon dispersion.
>
>
> HTH...
>
> Alex
>
>
> On Mon, Feb 10, 2020, 2:23 PM BHATTI Asif <Asi...@ls...> wrote:
>>
>> Hello,
>>
>> I have a disordered structure and want to know the stability and thermal properties of the structure and this can be obtained through vibrational analysis.
>>
>> I optimized my structure with VASP until the forces were E=-1E-4. I used DFPT to obtain the Hessian matrix (for this aproach we don't need to build larger supercells). When I checked the OUTCAR there were three imaginary modes (the last three can be ignored because they are translationary modes). Now when I generate the FORCE_CONSTANTS using phonopy it takes into account these modes which can be seen in the DOS and BAND structure.
>>
>> Should I remove these imaginary translationary modes before submitting them to Phonopy? vasprun.xml doesn't display any imaginary modes only OUTCAR does.
>>
>> Is this the right procedure? The acoustic soft modes are flat near the Gamma point.
>>
>> NB: I am attaching a file so you can see where the problem is.
>>
>> Regards,
>> Asif
>>
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>
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-- 
Atsushi Togo