[Pw_forum] Phonon atomic displacement units

[Eric Abel]

Hello,

I have a question about the units of the atomic displacments in the output 
of matdyn.x.  This issue has been addressed before, and I have read over 
those threads, but I have a add-on question.  First I assume that matdyn.x 
has the same output units as ph.x, which is 1/m^(1/2), where m is the mass 
in atomic rydberg units.  This is fine and dandy, however, I was wondering 
how to get from this to a unit of length, as in the end, that is the meaning 
of the atomic displacement eigenvector.  Perhaps it would help to indicate 
that what I would like to do in the end is calculate a inelastic x-ray 
scattering cross section which to first order goes like modsquared[Sum on i 
{(Zi*exp(q * ui)}].  Clearly ui needs to have the units of length in order 
to get the proper phase in the exponential.

Thank you in advance for your advice.

Eric