[QE-users] Phonons on a flat potential energy surface

[Antoine Jay]

Thank you very much.
I will try this and return the results (at least 1 week of calculation).
For the 'bad' ASR, there is the problem that the defect modes are so close to the acoustic modes that there is a hybridization between them.
I suppose that it should drastically affect the ASR.

Regards,
Antoine Jay

Le Samedi, Octobre 30, 2021 11:46 CEST, Lorenzo Monacelli <lorenzo.monacelli at roma1.infn.it> a écrit:
 Dear Antonine Jay,

By looking at your frequencies, they seem not well converged. Consider
that in a perfectly converged calculation the ASR should be respected
and its application should affect only transnational modes. However, all
the phonon frequencies change considerably there. Consider increasing
the tr2_ph value to 10^-20 or higher in the ph.x input script and
eventually also the wfc/density cutoff. Converging soft modes when a lot
of atoms and few symmetries are present in the system could be really
cumbersome with DFPT.

Bests,

Lorenzo


On 29/10/21 19:58, Tamas Karpati wrote:
> Dear Antoine Jay,
>
> I think that in case of multiple imaginary eigenvalues the lower
> energy of a conformer
> may be less indicative. On the other hand, frozen phonons might be
> both needed and to be avoided,
> unless you consider a sufficient number of surrounding atoms. Have you
> checked if inclusion
> of more atoms helps the imaginaries fade away? Did you try to toss O where the
> imaginary eigenvectors suggest? This way imaginaries can be sorted out
> one after the other
> (lucky ones get rid of them at once :)
>
> Hope this helps,
> t
>
>
>
>
> On Fri, Oct 29, 2021 at 10:10 AM Antoine Jay <ajay at laas.fr> wrote:
>> Dear users,
>> I would like to perform a phonon calculation at Gamma on a 216-atoms crystalline silicon supercell in which has been added 1 single oxygen (PBE functional).
>> This interstitial O is located 'nearly' in the middle of two Si, and unfortunately, many local minima are possible for its position: C2, C1, C1h, D3d, each of them being separated by a very small energy barrier and a small distance as already shown in Countinho-2000:
>> https://sci-hub.mksa.top/10.1103/physrevb.62.10824
>>
>> When I calculate phonons of the C1h (that has the smallest energy) within DFPT, I obtain imaginary eigenvalues in the dynamical matrix.
>> Of course, when I look at the corresponding eigenvectors, I see that they are exactly located on the O.
>> Then I manually do a frozen phonon calculation using a displacement equal to the eigenvector and I see a positive curvature of the parabola meaning that the system is well at a minimum.
>> Increasing the size of the displacement in my Frozen phonon permits me to see the double minima switching between C2 and C1h.
>> I suppose that the size of the perturbation in DFPT is too high...? Where can I change it is the code?
>>
>> Best regards,
>>
>> Antoine Jay
>>
>>
>> Inputs in attachment
>> Results:
>> Forces are over converged: Total force = 0.00049 Ry/au
>> Frequencies after DFPT are these
>> freq ( 1- 1) = -118.6 [cm-1] --> ?
>> freq ( 2- 2) = -114.5 [cm-1] --> ?
>> freq ( 3- 3) = -99.2 [cm-1] --> ?
>> freq ( 4- 4) = -38.0 [cm-1] --> ?
>> freq ( 5- 5) = -37.7 [cm-1] --> ?
>> freq ( 6- 6) = -37.7 [cm-1] --> ?
>> freq ( 7- 7) = 87.7 [cm-1] --> A' I+R
>> freq ( 8- 8) = 91.5 [cm-1] --> A' I+R
>> freq ( 9- 9) = 91.8 [cm-1] --> A'' I+R
>>
>> and after simple ASR:
>> # mode [cm-1] [THz] IR
>> 1 -107.94 -3.2359 0.0000
>> 2 -38.83 -1.1640 0.0000
>> 3 -10.69 -0.3206 0.0000
>> 4 -2.67 -0.0801 0.0000
>> 5 8.82 0.2643 0.0000
>> 6 29.09 0.8720 0.0000
>> 7 97.81 2.9324 0.0000
>> 8 98.94 2.9661 0.0000
>> 9 98.97 2.9670 0.0000 _______________________________________________
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