<html>Dear users,<br />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).<br />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:<br />https://sci-hub.mksa.top/10.1103/physrevb.62.10824<br /><br />When I calculate phonons of the C1h (that has the smallest energy) within DFPT, I obtain imaginary eigenvalues in the dynamical matrix.<br />Of course, when I look at the corresponding eigenvectors, I see that they are exactly located on the O.<br />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.<br />Increasing the size of the displacement in my Frozen phonon permits me to see the double minima switching between C2 and C1h.<br />I suppose that the size of the perturbation in DFPT is too high...? Where can I change it is the code?<br /><br />Best regards,<br /><br />Antoine Jay<br /><br /><br />Inputs in attachment<br />Results:<br />Forces are over converged: Total force = 0.00049 Ry/au<br />Frequencies after DFPT are these<br /> freq ( 1- 1) = -118.6 [cm-1] --> ?<br /> freq ( 2- 2) = -114.5 [cm-1] --> ?<br /> freq ( 3- 3) = -99.2 [cm-1] --> ?<br /> freq ( 4- 4) = -38.0 [cm-1] --> ?<br /> freq ( 5- 5) = -37.7 [cm-1] --> ?<br /> freq ( 6- 6) = -37.7 [cm-1] --> ?<br /> freq ( 7- 7) = 87.7 [cm-1] --> A' I+R<br /> freq ( 8- 8) = 91.5 [cm-1] --> A' I+R<br /> freq ( 9- 9) = 91.8 [cm-1] --> A'' I+R<br /><br />and after simple ASR:<br /># mode [cm-1] [THz] IR<br /> 1 -107.94 -3.2359 0.0000<br /> 2 -38.83 -1.1640 0.0000<br /> 3 -10.69 -0.3206 0.0000<br /> 4 -2.67 -0.0801 0.0000<br /> 5 8.82 0.2643 0.0000<br /> 6 29.09 0.8720 0.0000<br /> 7 97.81 2.9324 0.0000<br /> 8 98.94 2.9661 0.0000<br /> 9 98.97 2.9670 0.0000</html>