Mutual-nearest-neighbor pairs in fluids

The single solvent molecule coupled most strongly to a solute can play a special role in solute relaxation process in fluids whenever this special partner is itself sufficiently decoupled from other solvents. In atomic fluids this situation occurs whenever the solute and solvent form mutual-nearest-neighbor pairs. We use simulation here to calculate the equilibrium distribution of such pair distances and show that this distribution is well accounted for by a simple mean field theory. A look at the lifetimes of these pairs shows that they maintain their identity in rare-gas liquids for ∼100 fs.