Dear Prof. Gale,
I am using gulp to fit potential parameters.And I want to stop optimization if the objection function(i.e sum of square residues) change is belown a certain value.
But I find the unit of keyword gtol is fractional.Is that mean

Apologies but the units for ftol aren’t quite correct, since it’s a bit more complicated. The units for optimisation are eV, but for fitting they are mixed since observables can all sorts of different units. Coordinates might be fractional, but other things will be in their default units as input to GULP. With fitting, the value of ftol depends on the weights and so it’s hard to predict where you might want it to stop. Better just to limit the number of cycles if you want to stop before the end.