The Molecular Model and Its Solution

Consider first an adsorbing crystal, the surface of which provides M adsorbing sites for molecules of type A. We make the following assumptions  

The interaction between the site and the adsorbed molecule is characterized by a single parameter U. This is defined as the energy change for bringing A from a fixed position at infinite separation from the crystal to a specific site on the surface of the crystal. 

The sites are identical in the sense that they are all characterized by the same parameter, U. 

Neither the internal degrees of freedom of the crystal nor those of A are affected by the adsorption process. 

The sites are distinguishable i.e., different configurations of the molecules on the M sites are different quantum-mechanical states. 

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