United Atom Approach Research Articles

Dielectric Constant and Structure of Liquid 18-Crown-6 Calculated from Molecular Dynamics Simulations

The results are presented for molecular dynamics simulations of liquid 18-crown-6 using different potential models. The results offer the possibility of investigating the influence of the flexibility of the dihedral angles and the effects of the united atom approach. The radial distribution functions and the correlation between the molecular separations and relative orientations are found to be rather insensitive to the specific potential model used. The relation between orientation correlations and dipole−dipole correlations on the other hand is found to be very sensitive to the flexibility of the molecule. The contributions of the dipole−dipole correlations to the dielectric constant are found to be small compared to those of the molecular dipoles. The calculated dielectric constants are very much in disagreement with the experimental one. It is believed that adding electronic polarization terms to the potential models will very much enhance the contributions of the dipole−dipole correlations to the dielectric constant without necessarily changing the molecular and structural properties.

Molecular orientational motion in PCH5: computer simulations and models

The tumbling molecular motion in mesomorphic systems in discussed, based on a molecular dynamics computer simulation of a typical mesogen PCH5 within the united atom approach. It is shown that conformational molecular flexibility plays an important role not only in the orientational distribution of the molecules in the mesophase, but also in their dynamics. Being subjected to strongly fluctuating intermolecular forces, the relative position of the atoms in the molecule exhibit relaxation in the same time-scale as the intermolecular torque, which is much shorter than expected from the corresponding molecular moments of inertia. This results in a very fast short-time decay of the reorientational autocorrelation functions of the molecule fragments. In spite of this rather complicated picture, cage models can be used to fit the reorientational autocorrelation functions. For a more precise fit to the experimental functions one has to account for the real distribution function of the cage potential curvature.

Molecular Dynamics Model of Dimethyl Ether

We report a molecular dynamics model of the monomeric liquid dimethyl ether. The united atom approach is used to treat CH{sub 3} groups as point source centers. Partial charges are derived from the experimental dipole moment. Harmonic force constants are used for intramolecular interactions, and their values are so chosen that the model`s fundamental frequencies agree with experimental results. Because we are interested in solvation properties, the model contains flexible molecules, allowing molecular distortion and internal dynamical quantities. We report radial distribution functions and the static structure factors as well as some dynamical quantities such as the dynamical structure factor, infrared absorption, and Raman scattering spectra. Calculated results agree reasonably well with experimental and other simulation results. 25 refs., 8 figs., 1 tab.

A model for K-vacancy production in asymmetric ion-atom collisions

A model for K-vacancy production in ion-atom collisions is proposed. The model represents a simplification of the work of Briggs and Macek (1972) for symmetric collisions. It is based on a perturbed united-atom approach and applied also to asymmetric collisions. The agreement with experiment is as good as for the Briggs-Macek theory for Ne+-Ne collisions.