  We present a new linked cluster expansion for calculating properties of
multiparticle excitation spectra to high orders. We use it to obtain the
two-particle spectra for systems of coupled spin-half dimers. We find that even
for weakly coupled dimers the spectrum is very rich, consisting of many bound
states. The number of bound states depends on both geometry of coupling and
frustration. Many of the bound states can only be seen by going to sufficiently
high orders in the perturbation theory, showing the extended character of the
pair-attraction.
