compute xrd and supercells

Hi all - I’m trying to understand compute xrd, but I’m confused by how it handles supercells. From the expressions in
http://lammps.sandia.gov/doc/compute_xrd.html#xrd-coleman
it looks like if you compute the xrd for a monatomic cell, and for (for example) a 2x1x1 supercell, the structure factor of the supercell will be
F(k) = F_0(k) exp(2 pi i k . R)
where F_0(k) is the structure factor of the primitive cell and R is the supercell repeat distance.

This doesn’t look like it’s invariant, but I’d hope that the structure factor for a structure and its supercells should be the same no?

Is there something explicit done for supercells that I’m missing? Is this just some magic of k-space sums that’s not apparently obvious? Is this explained someplace in the Coleman reference that I missed?

thanks,
Noam

Hi Noam - I suggest you send an email to the author

of those computes, Shawn Coleman. He’s listed

in src/USER-DIFFRACTION/README.

Steve