The interaction energy of a cationic complex A-B+ can be computed as the sum of the interaction energy of the neutral complex A-B and the geometry dependent difference in the ionization potentials of the complex A-B and the molecule B, with ionization potentials calculated by the outer valence Green's function method. We test this method by computing the intermolecular potential energy of the complexes He-CO+, Ne-CO+, and Ar-CO+ for linear and T-shaped geometries. One-dimensional potential energy cuts were analyzed with emphasis on the asymptotic behavior. Results obtained by this method have been compared to interaction energies of the A-B+ complex computed directly by the partially spin-restricted single and double excitation coupled cluster method with perturbative triples. For the weakly bound complexes He-CO+ and Ne-CO+ the differences are only a few percent at small intermolecular distances but become significant for separations around the equilibrium distance and larger. Scaling the long range induction coefficients to match accurately known values significantly improves the agreement: the resulting interaction potentials are accurate to within a few percent at all intermolecular separations. For the Ar-CO+ complex the method produces less accurate results for small intermolecular distances but the binding in Ar-CO+ is very strong and for small R this system cannot be considered a weakly bound complex anymore. (C) 2003 American Institute of Physics.