On the accuracy of density-functional theory exchange-correlation functionals for H bonds in small water clusters. II. The water hexamer and van der Waals interactions

摘要: Second order Moller–Plesset perturbation theory at the complete basis set limit and diffusion quantum Monte Carlo are used to examine several low energy isomers of water hexamer. Both approaches predict so-called prism be lowest isomer, followed by cage, book, cyclic isomers. The energies four very similar, all being within 10– 15 meV/ H2O. These reference data then evaluate performance density-functional exchange-correlation xc functionals. A subset functionals tested for smaller clusters I. Santra et al., J. Chem. Phys. 127, 184104 2007 has been considered. While certain do a reasonable job predicting absolute dissociation various coming 20 H2O, none correct energetic ordering nor does any total isomer. All either book or have largest energies. many-body decomposition interaction hexamers leads conclusion that failure lies in poor description van der Waals dispersion forces It is shown addition an empirical pairwise attractive C6R �6 correction allows improved hexamers. relevance these results simulations liquid also briefly discussed. © 2008 American Institute Physics. DOI: 10.1063/1.3012573