The ABINIT Users Mailing List ( CLOSED )

[Philippe Ghosez <Philippe.Ghosez@ulg.ac.be>]

Hi,

As mentioned by Masayoshi, computation of the static
and IR frequency dielectric tensor can only by done
for stable structure (without imaginary frequency). To get
an estimate of the dielectric constant of cubic BTO, you
need indeed to consider anharmonic terms as it can
be done using a model Hamiltonian (as the one
developed by Zhong Vanderbilt and Rabe). Note also that
the dielectric response diverges at the ferroelectric phase
transitions so that to get a prediction comparable to
experiment at a given temperature you need a model
that properly predicts the phase transitions and their
temperatures. This can be achieved with the model
Hamiltonian (including a rescale of the temperatures).

The case of PbTiO3 is slightly different. When you do a
calculation in tetragonal PbTiO3, you will get from ABINIT
an estimation of the dielectric constant for this phase at zero
Kelvin. In this case, the value that can be compared to experiment
at room temperature  since PbTiO3 is tetragonal at room temperature
and you are still far away from Tc (above 600K)...

I hope that helps.

Philippe.

On 08 Feb 2006, at 00:52, Mustafa Uludogan wrote:

> Hi,
> I try to calculate the frequency dependent dielectric
> coefficients  for a typical cubic ABO3 ferroelectric
> compound
> by anaddb variables
>  dieflag   1     ! 2=> electronic dielectric tensor
> only
>  nfreq     100
>  frmin     0.0
>  frmax     0.008
> . I hav egotten the following output.
> If there are unstable modes present in the system, is
> there a way to calculate these properties?
>
> Yours
> Mustafa Uludogan
>
>
>
>
> Relaxed ion dielectric tensor
>
> phonon modes warning- :
> unstable eigenvalue detected in force constant matrix
> at Gamma point
> the system under calculation is physically unstabale.
>     -80.36253001      0.00013084      0.00008454
>       0.00013084    -80.35369361     -0.00006049
>       0.00008454     -0.00006049    -80.36250426
>
>  diel9 : ERROR -
>   The lowest mode appears to be a "true" negative
> mode,
>   and not an acoustic mode. This precludes the
> computation
>   of the frequency-dependent dielectric tensor.
>   Action : likely there is no action to be taken,
> although you,
>   could try to raise your convergence parameters (ecut
> and k-points).
>  For your information, here are the four lowest
> frequencies :
>  -7.95442387E-04 -7.95418229E-04 -7.95418229E-04
> 0.00000000E+00
>
>  leave_new : decision taken to exit ...
>
>
> Mustafa Uludogan
> Chemical Engineering Department
> Texas A&M University
> College Station, TX 77843
>
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--------------------------------------------------
Philippe GHOSEZ
Universite de Liege
Institut de Physique, Bat. B5
Allee du 6 aout, 17
B- 4000 Sart Tilman
BELGIUM

Phone : ++(32) (0)4-366.36.11
Fax   : ++(32) (0)4-366.29.90
E-mail: Philippe.Ghosez@ulg.ac.be

http://www.ulg.ac.be/phythema
--------------------------------------------------