>... to answer to your (too) long questions. May be later, OK?
Going back to this quartics versus ellipsoids peak broadening stuff, maybe I can summarise:
Why should the distribution of lattice parameters (=strain) in a sample match the crystallographic symmetry? If the sample has random, isolated defects then I see it, but if the strains are induced (eg: by grinding) then I'd expect the symmetry to be broken. Suggests to me the symmetry constraints should be optional, and that the peak shape function needs to know about the crystallographic space group and subgroups. Either the program or the user would need to recognise equivalent solutions when the symmetry is broken (like the "star of k" for magnetic structures).
Why would anyone have anything against using an ellipsoid? That same function can be described by the quartic approach, it just has less degrees of freedom.
In short, I don't understand why there is such a strong recommendation to use the quartics instead of ellipsoids or why the symmetry is not optional. I'm still persuing this because I have looked at something with a very small anisotropic broadening which seems to fit better with an ellipsoid which breaks the symmetry compared to a quartic which doesn't!
Thanks for any advice,
Jon
