  We examine the role of the condensing agent in the formation of
polyelectrolyte bundles, via grand-canonical Monte Carlo simulations. Following
recent experiments we use linear, rigid divalent cations of various lengths to
induce condensation. Our results clarify and explain the experimental results
for short cations. For longer cations we observe novel condensation behavior
owing to alignment of the cations. We also study the role of the
polyelectrolyte surface charge density, and find a nonmonotonic variation in
bundle stability. This nonmonotonicity captures two trends that have been
observed in separate experiments.
