[Ifeffit] restraints in larch

[Barbara Etschmann]

Hello

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.
nO1 = param(7.0, min=5, max=7, vary=True),
nO2 = param(1, vary=True),
 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
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