K. D. Jordan, F. F. Wang, G. Jenness, R. Kumar, and W. A. Al-Saidi
Department of Chemistry University of Pittsburgh Pittsburgh, PA 15260 USA

In recent years much effort has been dedicated to designing strategies for improving the performance of density functional theory (DFT) for treating weakly interacting systems. Much of the attention has focused on strategies for correcting DFT for long-range dispersion interactions. It is also becoming increasingly clear that it is essential to correct deficiencies in standard exchange functionals in order for the resulting DFT+vdW energies to correspond closely to the results of large basis set CCSD(T) calculations. In order to gain additional insights into the appropriateness of various density functional methods for describing water, we have used symmetry-adapted perturbation theory (SAPT) and energy decomposition analysis (EDA) to dissect the interaction energies of the low-lying ring, cage, prism, and cage isomers of the water hexamer. This analysis provides new insights into the performance of different density functionals for describing the exchange-repulsion, polarization, and charge-transfer contributions to the interaction energies.