Recently a computer adapted theory suitable for evaluating the dielectric properties of polar systems has been proposed. It rests on the assumption that the system behaves like a macroscopic dielectric and that the modifications of dipolar interactions do not affect its dielectric constant. In the present paper these assumptions are tested, for a system of 512 Stockmayer particles with μ*² = 3·0 and I* = 0·025 at ρ* = 0·822 and T* = 1·15, in a series of extensive simulations, in which the boundary conditions (spherical reaction field (RF) vs. lattice summation technique (LS)) have been varied in a systematic way. Within the limits of statistical accuracy all simulations give identical results for the static as well as for the frequency dependent dielectric constant, namely ε(0) = 66 and a relaxation time of τ_D* ≃ 1·0 for the almost Debye-like behaviour of ε(ω). For particle numbers of 256 and upward the system studied behaves like a macroscopic dielectric; the bulk dielectric constant is independent of N for RF as well as LS-geometry.

Two molecular dynamics simulations of 216 molecules interacting by the ST2 water potential were carried out. Apart from technical improvements, the first simulation was identical to that reported by Stillinger and Rahman [4]. In the second, however, the reaction field method was applied and the influence of long range interactions was carefully studied. 7500 time steps (= 7·5 ps) were generated. The temperature given by the average kinetic energy was 120°C. The atom-atom pair correlation functions and the average dipole energy are rather insensitive to the reaction field. The average cosine between two molecular dipoles, however, is strongly influenced and corresponding curves are of different sign in the second and third coordination shell. For the integral of the average cosine, namely the Kirkwood g-factor g _K, the situation is even worse : Large errors occur if the long range interactions are neglected. On the other hand, a convergence of g _K was achieved by the inclusion of the reaction field. The predicted g-factor, g _K = 2·3-3, as well as the dielectric constant, ε = 45-60, are in good agreement with experiment (g _K = 2·5, ε = 51). Finally, the single particle dynamics is analysed with the aid of the mean squared displacement and the reorientational cosine cos ϑ(t). For short times, corresponding curves are close together, but for longer times a maximum deviation of 20 per cent is observed.

In this study we present the results of the molecular dynamics simulation of the ionic liquids: 1-butyl-3-methyl-imidazolium tetrafluoroborate and trifluoromethylacetate as well as 1-ethyl-3-methyl-imidazolium dicyanamide. Ionic liquids are characterized by both a molecular dipole moment and a net charge. Thus, in contrast to a solution of simple ions in a (non-) polar solvent, rotational and translational effects influence the very same molecule. This study works out the theoretical framework necessary to compute the conductivity spectrum and its low frequency limit of ionic liquids. Merging these computed conductivity spectra with previous simulation results on the dielectric spectra of ionic liquids yields the spectrum of the generalized dielectric constant, which may be compared to experiments. This spectrum was calculated for the three ionic liquids over six orders of magnitude in frequency ranging from $10\mathrm{MHz}$ to $50\mathrm{THz}$ . The role of rotation and translation and their coupling term on the generalized dielectric constant is discussed in detail with a special emphasis on the zero-frequency limit. Thereby, the frequency dependence of the cross correlation between the collective rotational dipole moment and the current is discussed.