[Pw_forum] Sound velocity calculation

[Pavel B Sorokin]

Dear all!

I would like to calculate the sound velocity for graphene. It seems
rather simple but I have encountered a problem which I still can't
solve.
I have calculated the phonon band structure which seems reasonable
and have got the frequencies near gamma point (in cm-1).
Now I should just derivate omega (in s-1 = cm-1*3*10^10) by k and obtain velocity. But the
what units of k should I use?
The k units using in PWSCF is 2*pi/a0 where a0=4.66625 Bohr = 2.13
Ang and it seems correct.

The reciprocal lattice vectors are following:

 b(1) = (  1.0000  0.5774  0.0000 )
 b(2) = (  0.0000  1.1547  0.0000 )
 b(3) = (  0.0000  0.0000  0.1900 )

But when I try to evaluate the velocity I obtain the answer nearly in
six time less then reported early: 2200 m/s and 3400 m/s vs. 12200
m/s and 19500 m/s.
I do some stupid mistake, but where? Does anybody has the experience
in such calculations?

Thank you!!

-- 
Sincerely yours,
Pavel B Sorokin                         mailto:PBSorokin at gmail.com
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