How can I calculate of group velocity of guided mode?

Hassan Aghababaeian <[email protected]>
Newsgroups gmane.comp.science.photonic-bands
Message-ID <[email protected]>
I need to calculate group velocity of guided mode (or Group index) in some geometry like line defects. 
From froum I understood  display-group-velocities command in MPB can be useful. But I couldn't find what is output and how can be used?
When I extract Group velocities by: grep velocity *.out > result.dat, the output is some vectores like:
velocity:, 1, #(0.0 0.0 0.0), #(…
velocity:, 2, #(0.546472273423393 0.31550526440082 7.60657916462197e-9), …
 
What do these vectors mean? How can I calculate group velocity of guided mode?
 
Thanks
Hassan

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