Evaluation of the electrostatic osmotic pressure in an infinite system of hexagonally oriented DNA molecules

The electrostatic osmotic pressure in an assembly of hexagonally oriented DNA molecules is calculated using canonical Monte Carlo simulations. The calculations are performed within a continuum model for a system of seven uniformly charged DNA cylinders and balancing mono- or divalent counterions. We have relaxed the requirement of electroneutrality of the seven interacting one-DNA cells in our previous ‘interacting cell model’. The minimum image procedure, in conjuction with periodic boundary conditions, was used in all directions, and, by applying the virial equation, the pressure was evaluated for truly infinite systems. As found in the previous work, ion correlation effects are very important for systems with divalent counterions at short DNA separations. We find a force minimum and a net attraction at short separations. In addition, in systems with monovalent counterions at low dielectric permittivity there is a force minimum, also attractive in this case, but much broader and at rather large DNA separation.