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.