A covalency-electrostatics crossover sets the threshold of eleven water molecules for HF dissociation

≥ 11 is required to thermodynamically stabilize the ion-pair state, underscoring the key role of cooperative hydrogen-bond networks in breaking the H-F bond. Our energy decomposition analysis based on sobEDAw further reveals that the pronounced stability of the undissociated HF is primarily governed by covalent orbital interactions. We identify 1.17 Å as a critical threshold, beyond which the dominant stabilization mechanism switches to ionic electrostatic interactions, thereby revealing an electronic mechanism: a covalency-electrostatics crossover that sets the threshold of eleven water molecules for HF dissociation. By jointly establishing the size threshold, structural motifs and electronic driving mechanism for the dissociation of a weak acid in finite water clusters, this work provides a new theoretical basis for a unified understanding of weak-acid solvation and proton-transfer mechanisms at the molecular level.