  Water clusters are known to form through hydrogen bonding. However, this
study shows that the formation of very small water clusters significantly
deviates from this mechanism and instead involves both hydrogen bonding and
electron delocalization. Our density functional theory calculations show that
small water rings (H2O)n of n=3 or 4 show strong electron delocalization
originating from both the hydrogen and oxygen atoms and extending to the ring
center. This is very different from larger rings. Further energy decomposition
of rings with n=3-6 demonstrates an upward trend in the polarization component
but an decrease in the electrostatic and exchange repulsion components,
presenting a minimum and accounting for 33% of interaction energy at n=3. This
significantly promotes stability of the small water rings. Our findings provide
a comprehensive analysis and improve our understanding of the stability
characteristics of water clusters.
