[Pw_forum] doubt in ph.x and matdyn.x

[Paolo Giannozzi]

On Jan 18, 2010, at 9:13 , Bipul Rakshit wrote:

> when i plot the frequencies (*.freq file), given after running  
> matdyn.x file,
> using the above high symmetry points, the dispersion is all  
> positive (the
> frequencies are all positive)
>
> But when I am doing single q=0.3333 0.57735  0.0000 point  
> calculation in
> my graph.ph.in file. For this perticular q-point the frequencies  
> are -ve.
>
> Can anybody tell me the reason for this, or I am doing some error?

unless you have strong evidence of the opposite, I would say that the
latter point applies (unless the difference between the two cases is
small and related to violation of the Acoustuc Sum Rule)

P.
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Paolo Giannozzi, Dept of Physics, University of Udine
via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222