diff --git a/mrdna/model/arbdmodel.py b/mrdna/model/arbdmodel.py
index 8a7b75131fd3797eab4761f39b0df5100597905c..17afaf7e5fd14349c8e09cd399a5c881414ec215 100644
--- a/mrdna/model/arbdmodel.py
+++ b/mrdna/model/arbdmodel.py
@@ -1017,7 +1017,7 @@ tclForcesScript $prefix.forces.tcl
         if update_dimensions:
             format_data['dimensions'] = self.dimensions_from_structure()
 
-        for k,v in zip('XYZ', self.dimensions):
+        for k,v in zip('XYZ', format_data['dimensions']):
             format_data['origin'+k] = -v*0.5
             format_data['cell'+k] = v