The ABINIT Users Mailing List ( CLOSED )

[Konstantin Rushchanskii <konstantin.rushchanskii@cea.fr>]

Dear Pawel,

I can give only general suggestion.

In principle, the phonon frequencies obtained from "direct" ab initio 
calculations and from interpolation should coincide with reasonable 
tolerance, say, 0.5 or 0.1 cm-1. If not, it is an issue of a convergence.

1. Please check the acoustic sum rule and the charge neutrality in the 
output of anaddb file. In case a corrections are significant, try to 
increase improve your ab initio calculations.

2. Check the convergence with respect to density of q-point mesh, it can 
be crucial in strongly anisotropic systems.

3. If you deal with GGA, please note an significant error for acoustic 
modes (see  tread http://www.abinit.org/wws/arc/forum/2002-12/msg00016.html)

Hope this helps.

Regards,
Konstantin


Pawel Scharoch wrote:
> Dear Abinit users,
>
>   could you please give me a hint to the following problem.
>
>   I need to calculate phonon frequencies for a certain (say arbitrary)
>   set of k-points in BZ. I can do it in two ways:
>
>   - do it directly using main Abinit code, or
>   - calculate first force constant etc. (as described in the
>   tutorial).
>
>   Which way is expected to provide more accurate results ?
>
>
>   In fact, I have done such calculations but the two methods
>   gave slightly different results (thermodynamic
>   characteristics), unfortunately differing enough to lead
>   to different conclusions.
>
>   With best regards,
>
>   Pawel