Molecular dynamics calculation of the dielectric constant

Abstract

In the previous papers of this series it was shown, by molecular dynamics simulations, that 2D fluid systems of pure Stockmayer molecules have a higher dielectric constant than predicted by the Onsager model or, equivalently, that their Kirkwood orientational correlation factor g _k is greater than unity. In this paper this observation is further considered; it is shown that it is due to the fact that the longitudinal positive correlations are enhanced, while the negative correlations in the transverse direction are diminished, compared with those induced in a continuum by a polar molecule. Modified Stockmayer model molecules are then studied. The adopted modifications are of two kinds: (i) the molecular point dipole is excentred and (ii) a central quadrupole is added to the central dipole. In both cases g _k is found to be closer to unity than for the pure Stockmayer systems. However, this is not due to a better local validity of the Onsager picture: rather, the map of the local polarization around a reference molecule exhibits peculiar features which indicates that the polarization in certain approximatively transverse directions is more negative than for the pure Stockmayer system. This is caused by a closer approach of the dipoles in the first modification, and by the indirect influence of the quadrupolar interaction in the second one. Finally the dynamical behaviours of the different systems are compared: the multimolecular to monomolecular correlation times ratio behaves qualitatively as g _k.