Fitting gulp shell position

Is it possible to restrict shell positions while varying the charge in the fitting process?

If you mean “restrict” in the sense of the shell not moving then this is the default and so the shells will only move if you specify a keyword that causes them to move. It’s important to note that when fitting that the shells need to have no force acting on them at the end (as their position is unknown) otherwise the fit won’t be very meaningful.

I am only varying the charge through the usage of species
And variables split
But in the output file, I see the shel positions varying with final values

As mentioned before, you need to look at the keywords that you’ve specified as these control what happens to the shells. You need to check the manual/help text to understand the keywords you’ve given & what this means for the shells.

Thank you co much !