Hypotheses of the model configuration energy

We suppose that there is no short-distance order in the alloy, and therefore all of the configurational partition function is due to the long-distance order. 

All of the configuration energy is due solely to an interaction between near-neighboring atoms, and each atom A or B is surrounded 

For a given composition of the solution and in a given state r, we would have aa pairs A-A with energy,, bb pairs B-B with energy and ab pairs A-B whose energy is. For this state T, the configuration energy will, therefore be  

Using relation [2.45], this expression takes the form  

In the models developed below, the interaction energies will be taken to be independent of both the temperature and the interatomic distances or, if readers prefer, we suppose that these interatomic distances are not altered by the creation of order. 

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