Large-basis CI calculations are performed on the van der Waals complexes Arâ HCl and (H2O)2. It is shown that a reasonable estimate of the CI basis-set superposition error is obtained from a ghost calculation involving the orbitals of the monomer and only the virtual orbitals of the ghost.Both basis-set superposition error corrections and size-consistency corrections are of vital importance to obtain a reliable potential-energy surface.For Arâ HCl the minima of the potential are predicted within 50 µhartree of the experimental surface, viz. â 804 µhartree for the Arâ HCl orientation and â 565 µhartree for the HClâ Ar geometry.The water dimer van der Waals minimum is estimated to be â 4.9 kcal mol^{â 1}, which is less deep then experimentally derived minimum of â 5.4 kcal mol^{â 1}, but just within the experimental error limit.