Thank you Sir,
I have done few calculations of zone center phonons using primitive unit.
But i got wrong zone center phonon frequencies ( not all ) in LaB6 system.
Similarly in carbide systems.
e.g. For LaB6 system
Expt. freq
Freq (by SIESTA)
F1u mode ( translation) 210
181.12,182.30, 182.97
F1u mode (bending) 1095
877.12, 877.60, 879.85
and also
Accoustical
modes 0.0310,
0.0396, 0.0766,
Many times i changed the meshcutoff (kept more than 400 Ryd) and kgrid (
kept more than 25 Bohr) to get the required frequiences, But only the
digits after decimal place changes i.e meshcutoff and kgrid doesn't change
the frequencies appreciably.
Is the problem because of Pseudo-potential and basis.?
I also checked the frequencies using GGA approximation to particle density
functional, but no success?
hope that the information given is sufficient.
With Regards
1009u
On Sat, Dec 12, 2009 at 2:28 PM, <[email protected]> wrote:
> Dear 1009U,
>
> > I want to calculate the phonons at gamma point of the BZ.
> > 1) Do i need to make the supercell to find the phonons ?
>
> No
>
> > is a unit cell is sufficient for it?
>
> Yes
>
> > 2) which unit cell i mean whether primitive or conventional unit should i
> > use?
>
> For whatever cell, you'll get Gamma phonons of the Brillouin zone
> corresponding to your unit cell. Meaning, the primitive cell
> will give you the minimal number of modes, whereas the conventional cell
> will add some extra modes from the zone boundary of the primitive
> folded onto q=0 of the now smaller Brillouin zone.
>
> Good luck
>
> Andrei Postnikov
>
>
> > with regards,
> >
> > 1009U
> >
>
>
--
1009U
