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[Masayoshi Mikami <mmikami@rc.m-kagaku.co.jp>]

Hi, Mustafa,

As written in the error message, it cannot be computed...
The dielectric properties should be computed with
the most stable phase without negative phonon modes.

I just thought of  the cubic BaTiO3 (above 406K)...
In principle, frequency dependent dielectric properties
can be experimentally examined.
In this case, we need to think of the non-linear term
higher than 2nd order (with respect to atomic position delta_x)
to keep the BaTiO3 cubic.
e.g. suppose the 4th term,
        b (delta_x)^4 = b (delta_x)^2 * (delta_x)^2
from the thermodynamics (remember "1/2 kT" rule),
one (x^2) term may be approximately replaced with c (T).
Thus the 4th-order may contribute as B (delta_x)^2
(B: a phenomenological coefficient), thus the negative
coefficient of (delta_x)^2 may be positive from
the contribution of the 4th-order term (when T is high enough).

Such phenomenological term should contribute
to keep the BaTiO3 cubic above 406K, so that
the experiment about dielectric properties may be conducted.
Such discussion(implementation) would be possible
in the Density Functional Perturbation Theory in future ... ??

Regards,
Masayoshi

On 2006/02/08, at 8:52, 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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