[GSAS-II] Implementing constraints as in Topas

[Francisco Lieberich]

Dear GSAS-II community,

I recently had the opportunity to use Topas to do some Rietveld
refinements. One feature that seemed quite useful to me is the way
that constraints can be implemented. Instead of creating a constraint
equation one can type a variable name, such as x, into GUI fields
such as atomic position, site occupation, displacement parameter,
lattice parameter etc. Then if another field contains 1-x this
automatically creates the desired constraint equation (i.e. x + (1-x)
= 1). Is it feasible to implement similar functionality in GSAS-II?

Kind regards,
Francisco