I'm trying to use larch to refine the coordination of Y in solution. I need to use 2xO paths to fit the first shell and would like to restrain the total coordination to be between 5.5 and 8.

 

I can apply the constraint to each nO, but not the total, i.e.

 but

ntot1 = param(expr='nO1+nO2', min=5.0, max=8.5)

does not work.

How can I restrain the total coordination?

Also, having some problems with the gofs and errors - the chi square is rather small and often the errors are not reported and it is not clear why. 

Thanks

Barb