I would like to know whether GSAS is able to constraint the Uiso value
so that it will be always bigger than zero.
As others have indicated - GSAS does not do that (topas gives you the
option, doubtless amongst others). Shelx does do that (with single
crystal data) and you can't turn it off! Sometimes it is easier to find
out what is actually going wrong when parameters are not allowed to
refine to nonsensical values, and sometimes you might think you prefer
the values to be free. You can usually fix the uiso to reasonable values
and then try to work out what has gone wrong - didn't see anyone has
mentioning the lorentz-polarisation effects yet, which can also have a
great influence if you get the wrong one.
Good luck,
Jon
