[Pw_forum] phonon frequencies with pwscf and cpmd

[Massimiliano Bonomi]

Dear users,
I'm studying a crystal made of small clusters of eight atoms of sodium 
with FCC simmetry.
I'm using both pwscf and cpmd codes with the same parameters (cell 
dimension, cutoff for the wavefunctions, number of kpoints...),
but with two different pseudopotentials.
The two codes give the same results as far as the electronic part 
(energy levels) is concerned.
Then I started studying phonons: cpmd does not use a perturbation 
method, rather it moves ions along each axis in each direction
to calculate the forces and the hessian matrix. Here the two codes give 
very different results: the most striking difference is that pwsf's
phonon frequencies are not all positive, as you can see in the 
attachment, while cpmd's frequencies are all positive and quite 
different from the
previous ones.
For comparison I've calculated phonon frequencies with pwscf using 
car-parrinello finite differences method, moving manually each atoms. I 
found negative
frequencies which are in agreement with the ones I found with 
perturbation method.
How is it possible to explain these differences between the two codes???

Thanks in advance for your help.

Massimiliano Bonomi.

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