sommersj
I am a total newcomer to using TOPAS. Previously I had attempted to do this refinement in GSAS. I have neutron powder data for a hydrated inorganic oxysalt with numerous (deuterated) water molecules. positions of all non-D atoms are well-known by previous XRD work, so I am trying to focus only on refinement of the D positions.
My question: it is a good approach to first Distance_Restrain and/or Angle_Restrain the Ds of the water molecules to well-known O-H and H-O-H parameters, then finish by imposing AI_Anti_Bump to eliminate adverse contacts?
Much appreciate your opinion!
johnsoevans
It's hard to comment on the best approach without knowing the system or data quality.
In general you might initially try with water molecules as rigid bodies. This will ensure their geometry remains sensible. Perhaps start them with the O on the "known" sites to help locate D positions (i.e. just allow them to spin around the O site). Then allow the O coordinate to move.
You might then decide to test your answer by removing the rigid bodies and restraining the bond distance/angles to appropriate values. This is likely to be more discriminating than using an anti-bump (which just stops sites getting too close).
At the end you should test your model by either removing the restraints or lowering their weighting. This will give you an idea of how much you can trust your model.
Good luck.