New molecular dynamics simulation of a 3D fluid of Stockmayer and modified Stockmayer particles

Abstract

This paper is one of a series devoted to numerical simulations of polar fluids which do not employ periodic boundary conditions( PBC) but rather use a spherical isolated sample. In an earlier paper 2D fluids of particles interacting through 2D electrostatics were studied. We present here MD simulations using the same method for 3D fluids with the classical 3D dipolar interaction. Stockmayer particles are first considered, assuming reduced parameters which are close to those adopted by previous authors. Our results for various thermodynamic quantities are in good agreement with those of Pollock and Alder, who used MD with PBC and an Ewald summation, and of Patey et al. who used the MC method without Ewald correction. We have computed the static permittivity from the variation of the mean squared total moment of an inner sphere as a function of the relative volume of this sphere. We found ε=28, in agreement with the result of Pollock and Alder. This figure is much higher than that predicted by the simple Onsager model, and corresponds to a Kirkwood correlation factor g_k=2.73. The reason can be understood from maps of average polarization and local dipole correlations. On the other hand, the LHNC and QHNC results of Patey et al. yield a still larger permittivity, which is probably erroneous, as already noted by these authors. The dynamic properties of our system have also been studied—both the dielectric absorption–dispersion vs frequency and the orientational correlation functions are in fair agreement with those of Pollock and Alder mentioned above. We have made the same studies with Stockmayer‐like particles having their point dipole displaced along the symmetry axis from the Lennard‐Jones center (‘‘decentered Stockmayer particles’’); the dielectric constant thus obtained, ε=11, is much closer to that of the Onsager model, for reasons that are illustrated. Finally, we show that our model fluid of ‘‘decentered Stockmayer particles’’ has reduced parameters similar to those of CH₃F at 206 K. We propose a method for establishing a crude correspondence between the nonpolarizable fluids of the simulations and real fluids. Using this correspondence, our static result for the permittivity is in fair agreement with that of CH₃F. However, the results for the dynamics from the MD simulation correspond to a rotational friction which is too small, because of the lack of anisotropic steric forces in the model.