The Model and Its Partition Function

At equilibrium, the probability of finding the empty system in one of the two states is the same as the equUibrium mole fractions, and these can be read from the PF, 

We note that in Eq. (3.2.6) we have defined K in terms of the molecular parameters of the system (Ej and E ). By virtue of the equilibrium condition, K is also equal to the ratio of the two mole fractions X and X. In the subsequent sections we shall follow the changes of X and X upon binding of ligands, and we shall continue to use the parameter K as defined in terms of and Q. The equilibrium 

we turn to the PF of a system with ligands. There are altogether four states of our system, denoted symbolically by 

The four terms in the PF correspond to the four states of the system, as described symbolically in (3.2.8). 

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