In solution, the probability of an intermolecular base pair is proportional to the (concentration dependent) probability that a dimer is formed at all. Thus, it makes sense to consider the conditional pair probabilities given that a dimer is formed, or not. The fraction of structures without intermolecular pairs in our partition function Z (i.e. in the cofold model without initiation contributions) is ZAZB/Z, and hence the fraction of true dimers is