Accurate SCF calculations have been carried out to investigate the potential of interaction for pairs and triplets of water molecules. The most stable pair configuration involves a linear hydrogen bond of length ROO = 3.00 Å and strength 4.72 kcal/mole. Three-molecule nonadditivities are large in magnitude and vary in sign according to the hydrogen-bond pattern involved. In both aqueous liquids and solids, the net trimer nonadditivity effect amounts to increased binding energy, decreased neighbor distance, and slightly enhanced tendency toward perfect tetrahedral coordination symmetry. The nonadditivity furthermore is inconsistent with the phenomenology of simple mutual electrostatic polarization between neighboring molecules.

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