[Pw_forum] phonons

Nicola Marzari marzari at MIT.EDU
Sun Aug 20 01:16:48 CEST 2006

Dear Haiping,

there are two possibilities that lead to negative phonon frequencies at

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).


>> 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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