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