Slab z-translation when creating input files

Hi all,

I'm studying surface reactions with CASTEP. When creating the reactant and product structures, I usually copy and paste the reactant strucutre into a new document and change the position of one or more atoms at the surface to form my product structure. I then optimise this strucure before starting the LST/QST TS search. The problem is, when creating the input files for the product optimisation, Material Studio applies some very subtle geometry translation of the whole slab in the z direction (I assume to have the centre of mass at the centre of the box?). There is of course no problem is terms of energy but when using that structure into the preview tool, the program complains that the fixed atom do not have the same coordinates.  
Does anyone know how to turn that z-translation off?
There is little point using Material Studio if I have to edit my *.cell file by hand before running them...

Thanks
Ludovic