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Understanding dielectric spectra can reveal important information about thedynamics of solvents and solutes from the dipolar relaxation times down toelectronic ones. In the late 1970s, Hubbard and Onsager predicted that addingsalt ions to a polar solution would result in a reduced dielectric permittivitythat arises from the unexpected tendency of solvent dipoles to align oppositeto the applied field. So far, this effect has escaped an experimentalverification, mainly because of the concomitant appearance of dielectricsaturation from which the Hubbard-Onsager decrement cannot be easily separated.Here we develop a novel non-equilibrium molecular dynamics simulation approachto determine this decrement accurately for the first time. Using athermodynamic consistent all-atom force field we show that for an aqueoussolution containing sodium chloride around 4.8 Mol/l, this effect accounts for12\% of the total dielectric permittivity. The dielectric decrement can bestrikingly different if a less accurate force field for the ions is used. Usingthe widespread GROMOS parameters, we observe in fact an {\it increment} of thedielectric permittivity rather than a decrement. We can show that thisincrement is caused by ion pairing, introduced by a too low dispersion force,and clarify the microscopic connection between long-living ion pairs and theappearance of specific features in the dielectric spectrum of the solution.

Kinetic and ion pairing contributions in the dielectric spectra of electrolyte aqueous solutions