A new era of water research

As discussed in the preface and in Secs. 2.1 and 2.2, earlier models of water assumed that liquid water consisted of mixtures of components, one of which had the characteristic tetrahedral structure of ice. The tetrahedral packing of water molecules was introduced as an input into the theory. In 1972, a new era in water research began when a new model of water molecules was constructed in such a way that tetrahedral geometry of the packing of the water molecules could be obtained as an output of the theory. The input consisted of a pair potential with a propensity to form tetrahedral structure. 

These two different points of view were explained in Sec. 2.5 in connection with the primitive and cluster-primitive onedimensional models for water. In the primitive model single water molecules are defined through their pair potential. The structure formed by these water molecules — clusters of HBed molecules — is a result of the specific pair potential. On the other hand, in the cluster primitive model, the structures — clusters of HBed molecules — are assumed to be a part of the description of the model, and the HBs are now part of the internal description of the clusters. 

As will be explained in the next section, the built-in tetrahe-drality of the water molecule, though necessary for the success of the 3-D model (but not for the 1-D or the 2-D models), is not sufficient. One should also design the pair potential in such a way that the principle will be implemented. At the time of constructing the Ben-Naim and Stillinger (BNS) potential in the late 1960s, this implementation was achieved by trial and error, whereas for the 2-D and the 1-D potentials as well as for the 3-D potential discussed in Sec. 2.7.4 it was achieved by design. 

The 3-D models are obviously more realistic than either the 2-D or the 1-D models. However, this closeness to reality should not necessarily mean that the 3-D model is more valuable for understanding water. 

The reason for this is simple. The properties of liquid water do not depend only on the pair potential (as is approximately true for the case of simple liquids). Higher-order potentials are very important in the determination of the eventual structure and properties of water. 

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