We derive a power expansion of the correlation energy of weakly bound systems within the random phase approximation (RPA), in terms of the Coulomb interaction operator, and we show that the asymptotic limit of the second- and third-order terms yields the van der Waals (vdW) dispersion energy terms derived by Zaremba–Kohn and Axilrod–Teller within perturbation theory. We then show that the use of the second-order expansion of the RPA correlation energy results in rather inaccurate binding energy curves for weakly bonded systems, and discuss the implications of our findings for the development of approximate vdW density functionals. We also assess the accuracy of different exchange energy functionals used in the derivation of vdW density functionals.