I am struggling to access the undeformed coordinates of nodes in particular node set.

I want to calculate the relative displacement between top and bottom nodes of RTV Gasket (is of circular shape). First, displacements of these nodes must be transformed to local coordinate system. This local coordinate system does not depend on element. hence, element level local coordinate system cannot be defined.

Therefore, I am accessing the undeformed coordinates for each face of the gasket. Then calculating the 3 vectors denoting the local coordinate system. Then putting these into a transformation matrix and transforming the displacement values. And finally taking the difference of top and corresponding bottom node transformed displacement values to get relative displacement value.

However, I am not able to access the undeformed coordinates. I tried following things:

Accessing the node coordinates from instance/assembly:

node = odb_obj.rootAssembly.instances[el_inst_name].getNodeFromLabel(nid)
node_coords = node.coordinates

Accessing the node coordinates from step:

node_coords = odb.steps['Preload'].frames[0].fieldOutputs['U'].values[0].instance.getNodeFromLabel(nid).coordinates

Both are giving the same coordinates which are not undeformed coordinates as I verified with INP file.

Additionally, these nodes are defined at the contact, so they may have been adjusted before the analysis by Abaqus (not sure though!).

How do I access the undeformed coordinates from ODB?

Thank you!