Hi,
Apologies if this has already been asked. I did a brief search and couldn't find anything but if there's already been a discussion of this, please let me know and I can read what's been said already. I'm working with Topas 5.
I have magnetic neutron data for a material with a high degree of preferred orientation. To account for this, I am refining a structural and magnetic unit cell together. I have a spherical harmonic (order 4) preferred orientation function in both phases, and the parameters are tied together to be equal. The space group of the structural phase is I4/m.
The magnetic phase is 25x the volume of the structural unit cell volume. It was obtained from Isodistort. It has magnetic space group 77.18.
When I attempt to run the file, I get the following error:
Non-linear least squares (c) 1992-2013 Alan A. Coelho
Loading xyz's for i4/m from file C:\topasa\sg\i4om.sg
Trying to load space group name, number etc... but no space group file.
This error suggested to me that the problem is there's no magnetic space group information for this space group, so to this end my initial effort was to try to get the right space group in the file.
However, at some point, I noticed that when I removed the spherical harmonic PO from the magnetic phase, the input file runs fine - nothing else needs to be changed. So actually the issue seems to be that the spherical harmonic PO is not compatible with this magnetic space group.
The reason I am posting is to ask:
Firstly, whether this is due to some physical constraint, or is simply a bug in the software?
Secondly, can anyone recommend a preferred orientation function which is likely to work better in this context?
Thanks in advance.