[Pw_forum] problem with calculating elastic constants in AlAs

[Ezad Shojaee]

Hi
i tried to use the linear relation between omega(k)/k (near Gamma point in TA branch of phonon band structure of AlAs) in the [000]-[100] direction, to calculate the sound velocity along [100] and finally estimate the elastic constant C_44 of it(AlAs):
 
v_transverse[100]=sqrt(C_44/density)
 
but i have orders difference with the experimental value(56.6 Gpa). I wonder if anyone(tried the same) could tell me what may be wrong in my calculation? I calculated the desired velocity as 506m/s (by the mean of the 6th example's results)
Thanx for any comment
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