[Pw_forum] problems with phonon dispersion

[W. YU]

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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