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(P-2)-H-2 van der Waals complex with recent diabatic potentials obtained from ab initio calculations by Klos [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. (C) 2003 American Institute of Physics.