How to define contact Node to node

Node to Node contact is not available as a pure 'Contact'.

But you can achieve this using CGAP/PGAP.

I believe there are a few tutorials and the documentation is clear enough.

But essentially you create a gap element, and assign a PGAP property defining u0 (gap aperture) and K0/K1 stiffness for open/close gap.

In this case you need to perform a NLSTAT loadstep.