The potentials of mean force of sodium chloride and sodium dimethylphosphate in water: An application of adaptive umbrella sampling

The potentials of mean force between sodium and chloride ions and between sodium and dimethylphosphate ions in aqueous solution are calculated by the probability ratio method using adaptive umbrella sampling Monte Carlo with a variety of simulation setups. The potential of mean force of sodium chloride is found to have only one minimum in contradiction to earlier molecular dynamics results with the same intermolecular potentials, but different boundary conditions. Our result in the region of contradiction is supported by large system size and free energy perturbation calculations. It is established that the difference is due to the respective boundary condition schemes employed. This result is to be regarded as a consequence of the potential function scheme employed and not necessarily as a statement about actual aqueous sodium chloride. A similar dependence of the existence of the second minimum on boundary conditions is observed for the sodium-dimethylphosphate PMF.