  Experiments have revealed correlation-driven behavior of DNA in charged
solutions, including charge inversion and condensation. This paper presents
calculations of a lattice-gas model of charge inversion for the adsorption of
charged dimers on DNA. Each adsorption site is assumed to have either a vacancy
or a positively-charged dimer attached with the dimer oriented either parallel
or perpendicular to the double helix DNA chain. The entropy and charge
distributions of these three species are calculated including the lowest order
fluctuation corrections to mean-field theory. We find that the inclusion of the
fluctuation terms has a significant effect on the entropy, primarily in the
regime where the dimers are repelled from the DNA molecule and compete with the
chemical potential in solution.
