Nonlinear constraints on fitting variables

GULP
1

Dear all.
I cannot figure out how to tell GULP to impose nonlinear constraints on fitting variables:
smelectronegativity
Ga 3.15 2.85 1.0 1.0 0.0 0.0 0 0 1 1
Here, I want to set the bounds for the parameters zeta (third number) and Qcore (fourth number) as zeta>0 and 0<Qcore<31. I don’t see how (or whether) I can use the constrain command for that.
Many thanks,

Roman

2

Dear Roman,
I’m afraid that there isn’t currently an option to put general bounds on parameters in GULP at present. Sorry. Keeping things positive (i.e. > 0) is possible for somethings via the “abs” keyword, which aims to keep things positive where parameters going negative isn’t very sensible. In my experience of fitting, if a parameter tends to vary a lot and go to strange values then this means there is insufficient fitting data to constrain it. In which case, it’s best just to fix it (or add more/different data).
Best regards,
Julian