The theory needed to carry out calculations on atom-molecule van der Waals complexes containing open-shell atoms is developed. The discussion concentrates on complexes containing atoms in P states. Several possible expansions of the total wave function are described, and the matrix elements needed to construct the Hamiltonian matrix are set out. Several different angular momentum coupling cases may arise, analogous to Hund’s coupling cases in diatomic molecules. The bending energy levels of Ca-HCl, B-H-2, F-H-2, Cl-Cl-2, and F-N-2 are calculated, using simple models of the interaction potentials.