Effect of non-pairwise-additive interactions on bundles of rodlike polyelectrolytes

Like-charged rigid polyelectrolytes can attract each other due to counterion-mediated interactions to form bundles. However, the resulting effective interactions between rods are not pairwise additive. Here we calculate the free energy of an N-rod bundle explicitly. We find that the breakdown of pairwise additivity dramatically affects the stability of bundles of N rods and leads to a barrier in the free energy that grows with increasing salt concentration.