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 <a...@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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_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list users@lists.quantum-espresso.org
https://lists.quantum-espresso.org/mailman/listinfo/users
_______________________________________________
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list users@lists.quantum-espresso.org
https://lists.quantum-espresso.org/mailman/listinfo/users
