Numerical considerations in the computation of the electrostatic free energy of interaction within the Poisson-Boltzmann theory

We evaluate two different ways of calculating the contribution of the electrostatic stress to the free energy integral based on Sharp and Hönig's method within the finite difference nonlinear Poisson-Boltzmann equation method with the University of Houston Brownian Dynamics program. We show that only one of these approaches gives consistent results in the limit of zero ionic concentration for interactions of the order of magnitude of the hydrogen bond. The results are compared with results from both the linear Poisson-Boltzmann equation and the Debye-Hückel theory, for ion concentrations within the limits of validity of these approximate methods. We demonstrate this by application to DNA molecules.