  Using molecular dynamics simulations, it is demonstrated that monovalent
counterions can induce aggregation of similarly charged rod-like
polyelectrolyte chains. The critical value of the linear charge density for
aggregation is shown to be close to the critical value for the
extended-collapsed transition of a single flexible polyelectrolyte chain, and
decreases with increasing valency of the counterions. The effective interaction
potential between two rod-like polyelectrolyte chains is measured, and is
related to the angular distribution of the condensed counterions.
