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[Vincenzo Fiorentini <vincenzo.fiorentini@dsf.unica.it>]

p.s. : I meant 130i cm^-1, sorry.


On 9 Jun 2006, at 11:17 AM, Vincenzo Fiorentini wrote:

> Yup, I'd check Matthieu's second option. If you study, say, cubic  
> LaAlO3 in a cubic cell doubled along 111 you get a triplet at 130i  
> at k=0 which is a backfolded Raman mode related to a rotational  
> distortion turning LaAlO3 into rhombohedral. But if cubic symmetry  
> is imposed, of course you'll never get there.
>
> Vincenzo Fiorentini
>
> On 9 Jun 2006, at 11:06 AM, Matthieu Verstraete wrote:
>
> > This could be
> >
> > 1) a spurious phonon frequency coming from the pseudopotential.  
> > This is the subject of several old posts. Please check the  
> > archive. If you increase the ecut it should get smaller (not the  
> > solution though - get a better psp).
> >
> > 2) a true instability. You should analyse the mode anaddb is  
> > attributing to the negative frequency. As Pierre-Matthieu said,  
> > the forces are zeroed by abinit due to symmetry, and one could  
> > imagine a phase transition which breaks the symmetry (hence  
> > impossible to relax to). Give the atoms a small displacement along  
> > the phonon mode (provided it does break symmetry), and relax with  
> > ionmov 2 and optcell 2, to see what happens.
> >
> > Matthieu
> >
> > --
> > ================================================================
> > Dr. Matthieu Verstraete                 mailto:mjv500@york.ac.uk
> > Dept. of Physics, University of York,     tel: +44 1904 43 22 08
> > Heslington, YO10 5DD York, United Kingdom fax: +44 1904 43 22 14