Application of an exact two-structure model

In the next section, we shall discuss the application of an exact two-structure model based on the MM approach. 

The simplest version of the MM approach and the one that has most often been applied is the two-structure model (T5M). Again, we stress that the exact T5M does not involve any model 

Instead of the CN, we can use the number of HBs for our classification into species. For instance, we can consider the four HBed species as one species of the TSM and refer to the other species (with n = 0,1,2,3) collectively as a second species. 

As a second example, consider the quasi-component distribution function based on the concept of binding energy (BE). We recall that the vector (or the function) xbe gives the composition of the system when viewed as a mixture of molecules differing in their BE. Thus, XBE( )dv is the mole fraction of molecules with BE between v and v + dv. A possible TSM constructed from this function is 

The above examples illustrate the general procedure by which we construct a TSM from any quasi-component distribution function. From now on, we assume that we have made a classification into two components, L and H, without referring to a specific example. The arguments we use will be independent of any specific classification procedure. We will see that in order for such a TSM to be useful in interpreting the properties of water, we must assume that each component in itself behaves normally (in the sense discussed below). The outstanding properties of water are then interpreted in terms of structural changes, i.e. redistribution of the molecules into various species that take place in the liquid. 

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