Counterion-mediated, non-pairwise-additive attractions in bundles of like-charged rods

Stiff polyelectrolyte chains, such as DNA, can attract each other in soluti on even though they have the same sign of charge. The attractions are media ted by multivalent counterions, which lead to an effective interaction at t he two-chain level that is attractive at short range and repulsive at long range. However, the effective interchain interactions are not pairwise addi tive. We present a formulation that allows theoretical treatment of the man y-chain Problem without assuming pairwise additivity. We show that a bundle of chains held together by counterion-mediated attractions can be describe d in terms of a bulk and surface free energy, and discuss the temperature d ependence of the attraction. [S1063-651X(99)00907-1].