Density expansion for particle-particle correlations in time-dependent physical clusters

We present a theory for the particle-particle correlations in physical clus ters for which bonding between particles is determined by a connectivity di stance and a permanency time. A generalized Mayer density expansion for the cluster pair correlation function is found, as well as an Ornstein-Zernike like relation. We can rely on this formalism to study clustering in realis tic models by applying techniques of liquid state theory.