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[matthieu verstraete <matthieu.jean.verstraete@gmail.com>]

Joe is quite correct, there should only be 3 0-frequency modes.

Regarding negative modes, however, you can get some for a relaxed
structure, in case the atoms or cell are constrained by symmetry. If
this is the case, the symmetry gives the GS structure, but the
perturbations break the symmetry, and can access the negative second
derivative of Etotal(ionic coordinate). This implies the equilibrium
is unstable wrt the broken symmetry structure. Otherwise I think
negative phonons are excluded.

Basically, equilibrium structure means the first derivatives of Etotal
are 0, but says nothing about the second derivatives.

You should also check that the negative modes are physical, and not
the result of the pseudopotential etc... (in particular if they are at
gamma). See other forum posts on breaking the acoustic sum rule (ASR).

Matthieu

>>
>> In principle ,do I definitely get positive phonon frequency at
>> q=(0,0,0) after I relax the cell shape and ion position completely ? I met 
>> a
>> problem of getting imaginary frequency after the relaxation of both the 
>> cell
>> shape and ion position .
>> Thanks for hint.
>>
>> Yuanxun
>>
>>
>>
>> ________________________________
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-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete

European Theoretical Spectroscopy Facility (ETSF)
Dpto. Fisica de Materiales,
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Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500