Thank you Stefano and Nicola for your advice. I just want to close this issue, after almost three months calculating the phonons. I added random displacements to the atoms and relaxed the atomic position and the cell shape. In this way, the calculations were done without symmetry. The atomic positions and the cell shape changed a little bit, but the structure was not unstable.
Then, for each cutoff (30,40, and 50 Ry), I relaxed the positions and followed a Gamma-point phonon calculation. A summary is Cutoff 30 Ry 40 Ry 50 Ry smallest omegas -63,-45,14 -57,-27,-12 -44,-32,-9 largest omegas 637,691,692 640,691,692 639,692,692 (omegas in cm^-1) Hence, I consider that I have reasonable convergence, and even the awful negative frecuencies go towards 0. Was I think that is to be learnt, is that before a phonon calculation one must relax the positions after breaking the symmetry. The bad news is that the calculation becomes much more expensive. Best regards Eduardo Message: 3 Date: Thu, 20 Jan 2005 12:43:33 +0100 From: Stefano de Gironcoli <[email protected]> To: pw_forum at pwscf.org Subject: Re: [Pw_forum] Re: convergence of phonon Reply-To: pw_forum at pwscf.org There can be many reasons for appearance of negative (actually immaginary) frequencies in a stable system (insufficient convergence, poor k-point sampling, bad pseudopotentials, bug in the code...). But it is also possible that the calculation is telling you something about your system. Your forces are zero, most of them for symmetry reasons (judging from the fact that they are exacly zero!!!). It is possible that your system is not in stable equilibrium but in a saddle point (that can be physically relevant or due to poor k-point sampling, insufficient cut-off, bad xc-functional, bad pseudopotential, ...) Your negative fequencies are two-fold degenerate that means for sure that they do not belong to the totally symmetric irreducible representation of your crystal symmtry (the one that is preserved during relaxation). Try breaking the symmetry of your system moving the atoms along one ove the unstable eigenvectors or just add some random displacements to your coordinates and relax again . Does the system go back to the original position ? if this happens there is a problem in the phonon calculation. Or the system goes somewhere else ? If this happens your original positions actually correspond to a saddle point and the phonon code spotted it. I'm suspicious about your degauss, it looks very small to me. I would rather use a larger degauss and m-p or m-v smearing Make a plot of your DOS broadening it with FD with degauss=0.002 If your sampling is sufficiently dense the DOS will be smooth, otherwise it will wildy oscillate. best regards, Stefano de Gironcoli Eduardo Ariel Menendez P wrote: >Hello phonon community, >Thanks to nicola marzari for his advice on the parameters for phonon >calculation. >However, I have tested the convergence of the phonon >frequencies against the wavefunction cutoff, and I do not find convegence. >I am surprised that I obtain very large negative frequencies like > omega( 1) = -4.995406 [THz] = -166.629919 [cm-1] > omega( 2) = -4.995406 [THz] = -166.629919 [cm-1] > omega( 3) = -4.474172 [THz] = -149.243320 [cm-1] > omega( 4) = -4.474172 [THz] = -149.243320 [cm-1] > omega( 5) = -2.511886 [THz] = -83.788069 [cm-1] > omega( 6) = -2.511886 [THz] = -83.788069 [cm-1] > omega( 7) = 0.734339 [THz] = 24.495069 [cm-1] > omega( 8) = 1.858145 [THz] = 61.981460 [cm-1] > etc, up to omega(36) > >
