[Pw_forum] negative frequencies....

[Uri Argaman]

I know this subject has already been discussed a lot here.
But I did not understand yet the hole meaning of the negative frequency.
1. unstable structure should lead to imaginary frequency not a negative one.
    so how come it become negative in the phonon dispersion? is it related
to the square of it?
2. how can we distinguish between unstable structure and problems with an
convergence ones?
3. we have done a calculation of Ti hcp with q-point up to 2 2 2 and got no
negative frequencies
    and good agreements with older published calculations. After change it
to 4 4 4 q-points grid
    we got wiggles and negative frequencies.
4. Is there a simple way of being aware of Kohn anomalies or we must use
el-ph coupling for that?
thanks very much, Uri.
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