Potential energy curves for HeH, NeH, and ArH are determined using large basis sets and high levels of theory. The computed potential energy curves are in excellent agreement with the experimentally determined model potentials for HeH and NeH, but differ with the model potentials deduced for ArH by Tang and Toennies. We derive a potential based on the ab initio calculations and our best estimate of the well depth; using this potential we are able to reproduce the results of the orbiting resonance experiments of Toennies and co-workers.