This post is a collection of notes of tips and tricks for using GSAS-II.
Multi-phase refinement
When we have more than one phase in GSAS-II refinement, we should have phase fraction for each of the phases involved. With this regard,
we have, suppose we have two phases, three parameters – the overall scale factor and two phase fraction factors. In this case, we cannot
let all of these three parameters to be refined since otherwise the program will by a high chance crash due to the strong correlation among
these parameters. Instead, we should refine the scale factor together with one of the two phase fraction factors, while setting a constraint
to specify that overall phase fraction should be 1. To add the constraint, we should follow the instruction in Ref. [1] (see #3 in Step-I),
or here. Here it should be noticed that the phase fraction is referred to as Scale for each
phase and this Scale should not be confused with that of the overall histogram.
References
[1] https://subversion.xray.aps.anl.gov/pyGSAS/Tutorials/SeqRefine/SequentialTutorial.htm