Hexamers and witchamers: Which hex do you choose?

Water and argon hexamers are examined using correlated wavefunction methods, the effective fragment potential (EFP) method, and Hartree–Fock (HF) theory and several density functional theory (DFT) functionals. The HF and DFT methods have been employed both with and without dispersion corrections. The computationally inexpensive EFP method captures the high-level coupled cluster binding energy and relative isomer energy predictions very well for both types of hexamer, much better than the DFT methods (DFT-D), even those that include dispersion corrections. Interestingly, the dispersion corrected HF method (HF-D) does very well. When the DFT-D methods perform reasonably, they do so because of a fortuitous off-setting cancellation of errors two-body and many-body contributions to the binding energy. The HF-D method does not rely on such error cancellation, while the EFP method captures both two-body and many-body contributions to the water hexamer very well. The many body contribution to the argon cluster are small and are most likely due to dispersion.