We present a general derivation of the expansion of diabatic intermolecular potentials for an open-shell atom interacting with a closed-shell molecule and the multipolar expansion of these potentials in the long range. It is outlined how to compute bound states of the open-shell atom– molecule complex from the set of asymptotically degenerate diabatic potentials in a body-fixed basis of rovibrational wave functions with the inclusion of spin–orbit coupling. This method is applied to produce all the bound energy levels of the F(2 P) – H 2 van der Waals complex with recent diabatic potentials obtained from ab initio calculations by Kłos et al. Int. J. Quantum Chem. 90, 1038 2002. The binding energy D 0 is 14.6 cm 1 for the para-H 2 complex and 19.3 cm 1 for the ortho-H 2 complex. The para-H 2 – F complex does not possess any bound states for rotational quantum numbers J larger than 9 2 , the ortho-H 2 – F complex has a maximum J of 11 2 . © 2003 American Institute of Physics.