[Pw_forum] phonons

[lan haiping]

Dear  Prof Nicola,

Thanks for your long suggestion.

I may not follow some of your words, would you please clarify  ?
 1,  How to relax the ions more ?
 2, I knew from most refs that ecutwfs  30 Ry for ultrasoft-PP is enough in
pw calculations , Is it right ? But i donot have much idear about ecutwfs in
ph calculations .  Would you please give me some refs of more detail
settings about PH calculations ?


I am trying to find more problems related to this wrong result following
your suggestion.
Thank you again, it really helps me much.

p.s.  It is nice for me listening  your lectures in Beijing , i really learn
much from your 3 lectures .


Regards,

Hai-Ping


On 8/20/06, Nicola Marzari <marzari at mit.edu> wrote:
>
>
>
>
> Dear Haiping,
>
>
> there are two possibilities that lead to negative phonon frequencies at
> gamma.
>
> The first one is less likely:
>
> 1) you have not carefully relaxed the *internal* degrees of freedom -
> i.e. the ionic coodinates. Then, the phonon code discovers that moving
> the atoms along a certain pattern (an "eigenvector" of the dynamical
> matrix at gamma) the energy goes down. Perfect - just relax the ions
> more. Relaxation of the unit cell has nothing to do with this
>
> The second one is more likely:
>
> 2) the calculation is not converged with respect
> to the plane wave cutoff needed, and the number of k-points needed,
> for the small or zero frequencies at gamma. Obtaining an accurate,
> faithful description of low energy modes requires cutoffs and especially
> duals much higher than anything you are used to (to give you an example,
> for an ultrasoft C for diamond, you need 100 Ry in the wavefunction and
> 2800 Ry in the charge density to get a convergence, at gamma,
> of the zero modes down to the last cm-1). Does it mean you need
> to use 100/2800 Ry for diamond ? No ! By enforcing the translation sum
> rules that the interatomic force constants must satisfy, you can monitor
> (after enforcing said rules) the convergence of the low frequency modes,
> and discover you can use much more reasonable cutoffs.
>
> Still, even more than a regular calculations, phonons require a
> through control on the parameters that define your calculation.
>
> Note that, as a warm-up, it is easier to converge LDA calculations
> than GGA calculations (the exchange-correlation terms, calculated in
> real space, introduce very high frequencies in reciprocal space).
>
>                        nicola
>
>
> >> Dear All,
> >> I also have calculated gamma phonon frequencies of anatase TiO2 .
> >> I run ph.x caculation after relaxing the structure, but  9 of 18
> frequencies
> >> are minus.
> >>
> >> Would you please give me some hints and help?
> >>
> >> input settings are below, thank you in advance !
> >>
> >> *phonons of TiO2 at Gamma
> >>  &inputph
> >>   tr2_ph=1.0d-14,
> >>   recover=.true.,
> >>   epsil=.true.,
> >>   prefix='tio2',
> >>   fildyn='TiO2-gamma.dyn',
> >>   outdir='/home/haiping/tmp/'
> >>  /
> >> 0.0 0.0 0.0*
>
>
> ---------------------------------------------------------------------
> Prof Nicola Marzari   Department of Materials Science and Engineering
> 13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
> tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
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