The ABINIT Users Mailing List ( CLOSED )

[Xavier Gonze <gonze@pcpm.ucl.ac.be>]

Dear Jorge,

Jorge Iniguez wrote:
> Dear abinitioners,
>
> This email is a (long-due) follow up to an early-August discussion on
> phonon calculations for MgB2. (Sorry for the delay. Summer is summer!)
> The problem then was that I was not getting the right degeneracies for the
> phonons (even though abinit was recognizing the right symmetry). As Lu
> Fu-Fa pointed out, the solution was to specify kptopt=2 in the Gamma
> phonon calculation.  I do not yet understand why kptopt=2 solves the
> problem (which is probably related with the construction of the dynamical
> matrix from an irreducible set of distortions), but OK, it works now.

The use of symmetries to reduce the number of k points is implemented
differently  in the ground state case and the linear response case, in ABINIT.

Indeed, for the ground state, you know that you have to work in the
irreducible Brillouin zone defined by ALL the symmetries. However, for
most perturbations, not all the symmetries of the unperturbed cell will
be preserved ! So, in the case of response function, the user has to specify
the full set of k points (except, possibly, for the gamma point, where the
time-reversal symmetry is always preserved), and the code will take care
of the folding of the k points in the perturbation-adapted irreducible part 
of the
Brillouin zone, for each perturbation in turn.

This is actually described in the respfn help file :
http://www.abinit.org/ABINIT/Infos_v4.1/respfn_help.htm#3


> So, it seems there is
> something funny about the acoustic phonons in the metallic case. I guess
> this is not an input-file problem...  (a typical input file goes
> attached).  Any suggestions?

Interesting case !
I will try to look at it, after the v4.2 merge ...

Xavier