An explanation of the unusual strength of the hydrogen bond in small water clusters.

We seek to explain why the hydrogen bond possesses unusual strength in small water clusters that account for many of the complex behaviors of water. We have investigated and visualized the donation of covalent character from covalent (sigma) to hydrogen bonds by calculating the eigenvector coupling properties of quantum theory of atoms in molecules (QTAIM), stress tensor σ(r), and Ehrenfest Force F(r) on the F(r) molecular graph. The next‐generation three‐dimensional (3‐D) bond‐path framework sets are presented, and only the F(r) bond‐path framework sets reproduce the earlier finding on the coupling between covalent (sigma) and hydrogen bonds that possess a degree of covalent character. Exploration of the bond‐path between the covalent (sigma) and hydrogen bond's critical points provides an explanation for the previously obtained coupling results. The directional character of the covalent (sigma) and hydrogen bonds' 3‐D bond‐path framework sets for the F(r) explains differences found in the earlier results from QTAIM and the stress tensor σ(r). [ABSTRACT FROM AUTHOR]