  We consider a chemical reaction network governed by mass action kinetics and
composed of N different species which can reversibly form heterodimers. A fast
iterative algorithm is introduced to compute the equilibrium concentrations of
such networks. We show that the convergence is guaranteed by the Banach fixed
point theorem. As a practical example, of relevance for a quantitative analysis
of microarray data, we consider a reaction network formed by N~10^6 mutually
hybridizing different mRNA sequences. We show that, despite the large number of
species involved, the convergence to equilibrium is very rapid for most
species. The origin of slow convergence for some specific subnetworks is
discussed. This provides some insights for improving the performance of the
algorithm.
