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

[Antoine Jay]

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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