Symmetry breaking in water molecule interactions

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Abstract
We discuss the phenomenon of symmetry breaking observed in minimum−energy structures for water dimers. Based on two explicit representations of the intermolecular pair potential, the ST2 and PKC forms, lowest energy dimers are evaluated as a function of oxygen−oxygen separation, and it is concluded that a critical distance Rc exists which separates mechanically stable dimers of differing symmetries. For small distances, the familiar linear hydrogen bond is manifested, while for large distances dipole−dipole forces promote alignment of the molecular axes. At Rc, eight equivalent small−distance minima of low symmetry and four large−distance high−symmetry configurations undergo confluence. Thorough investigation of constrained minima illustrates how this transition occurs and invites comparison with the theory of critical phenomena. The second−order phase transition of the Ising ferromagnet provides a useful analogy to the structural symmetry breaking of the water dimer. Finally, we consider the relation of the present case to the larger issue of interactions in condensed phases, and to the interaction of more complex molecular pairs; one thereby discovers analogs for (a) Ising models in external fields, (b) upper and lower consolute points in liquid mixtures, and (c) first−order phase transitions.