[Pw_forum] problems with phonon dispersion

[Eyvaz Isaev]

Hi,

Did you to calculate phonons for Na using these
pseudopotentials? 

Bests,
Eyvaz.

--- "W. YU" <yuwen_66 at yahoo.com> wrote:

> Dear all,
> 
> I met some problems with phonon calculations and I
> hope someone could give me some help.
> 
> I did some calculations on a system with NaCl
> structure. I used three types of pseudopotentials:
> norm-conserving LDA, ultrasoft LDA, ultrasoft GGA.
> The
> lattice constants are all in good or reasonable
> agreement with experiments or other full potential
> calculations. When it comes to phonon dispersion
> curves, the NC LDA pseudopotentials gave smooth
> curves
> and the agreement with experiment is relatively
> good.
> 
> For the US LDA calculation, the dispersion curves
> are
> smooth and the agreement with experiment is
> acceptable, but the smallest achievable values of
> the
> accoustic branches at Gamma point are about 40
> wavenumber. With the increase of the ecut and
> ecutrho,
> these values became as large as 70 wavenumber. Now
> my
> question is: aren't they supposed to go to zero with
> the increase of ecut and ecutrho? If the answer is
> yes, does this mean the pseudopotential has some
> flaw
> or it is completely untrustable?
> 
> As for the US GGA, I found negtive frequencies
> around
> gamma point with the same ecut and ecutrho as the
> LDA
> case. So I used larger ecut and ecutrho, the negtive
> frequencies became positive, but there are some
> kohn-like anomalies in the accoustic branches and
> the
> agreement with experiment became very poor for the
> accoustic branches. I though this might be caused by
> long range interactions. So I took a 888 q point
> grid
> instead of the original 444 one. This time, besides
> negtive frequencies around the gamma point and the
> anomalies, the accoustic branches even became
> zigzaged! I really couldn't figure it out. Does
> anybody has similar experience? Could anyone tell me
> what could be the most possible cause for this? 
> 
> PS: accoustic sum rule has been imposed throughout
> these calculations. ONLY the accoustic branches have
> these problems. The optical branches seem to be
> insensitive to the changes of the q point grid and
> cutoffs.
> 
> Thanks a lot,
> 
> W. YU
> 
> 
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