Hi, while working on a really fast way to calculate the structure factors of a few millions of different modifications of a structure with only the selected scatterers contributing to f_calc I came across this: Why are there two methods for changing the sites_(cart/frac) of the scatterers doing quite different things? set_sites_... seems to simply update the corresponding site information of the scatterer, while replace_sites_... seems to create a copy of the structure. Is this the only difference there is? Also If anybody got a nice idea for my original problem, I'm also very much interested. (I think building a larger structure and working with selections might be faster than creating lots of different structures containing only the desired scatterers, but I might be wrong on this.) Cheers, Jan