Accurate modeling of complex systems relies on a correct description of van der Waals interactions. To understand their performance, we have examined six density functionals (B3LYP, TPSS, HCTH407, PW91, PBE1PBE and B971) and the MP2 method for their ability to predict binding energies and separations for 28 van der Waals dimers, involving dispersion, dipole-induced dipole, dipole-dipole and hydrogen-bonded interactions. The accuracy of all methods improves nearly monotonically with increasing strength of interactions. Of the functionals tested, counterpoise-corrected B971/6-311 ++G(3df, 3pd) has the best overall performance for binding energies, with a mean absolute percent error of 29.9% compared to 18.4% for MP2. Crown Copyright (c) 2005 Published by Elsevier B.V. All rights reserved.