Equilibration strategies in molecular dynamics simulation of polyethers

A comparison of the NVT and NVE ensemble simulations of tetraglyme showed that, in terms of energy, temperature and most of the structural features the results were very similar. However, major differences were observed in dynamic properties, ie in the mean square displacement and in the O--O distances. A fast equilibration method suitable for amorphous polymer systems is also detailed. This was accomplished by the reassignment of the velocity distribution to the particles, after the interruption of an NVE simulation.

Molecular dynamics simulations of highly concentrated salt solutions : structural and transport effects in polymer electrolytes

Structural, thermodynamic and transport properties have been calculated in concentrated non-aqueous NaI solutions using molecular dynamics simulations. Although the solvent has been represented by a simplistic Stockmayer fluid (spherical particles with point dipoles), the general trends observed are still a useful indication of the behavior of real non-aqueous electrolyte systems. Results indicate that in low dielectric media, significant ion pairing and clustering occurs. Contact ion pairs become more prominent at higher temperatures, independent of the dielectric strength of the solvent. Thermodynamic analysis shows that this temperature behavior is predominantly entropically driven. Calculation of ionic diffusivities and conductivities in the NaI/ether system confirms the clustered nature of the salt, with the conductivities significantly lower than those predicted from the Nernst-Einstein relation. In systems where the solvent-ion interactions increase relative to ion-ion interactions (lower charge or higher solvent dipole moment), less clustering is observed and the transport properties indicate independent motion of the ions, with higher calculated conductivities. The solvent in this system is the most mobile species, in comparison with the polymer electrolytes where the solvent is practically immobile.

Highly concentrated salt solutions : molecular dynamics simulations of structure and transport

Molecular dynamics (MD) simulations in NaI solutions, where the solvent has been represented by the Stockmayer fluid, were performed as a function of temperature, salt concentration, and solvent dipole strength. At higher temperatures contact ion pairs become more prevalent, regardless of solvent strength. An examination of the temperature dependence of the potential of mean force demonstrates the entropic nature of this effect. The transport properties calculated in the simulations are dependent on the balance between solvent dielectric constant and ion charge. In systems with a large solvent dipole moment, the ions appear to be independently mobile, and deviations from Nernst–Einstein behavior are small. In systems of smaller solvent dipole moment or greater ion charge, the ions form clusters, and large deviations from Nernst–Einstein behavior are observed.

Ion clustering in molecular dynamics simulations of sodium iodide solutions

Model systems of sodium iodide dissolved in dimethyl ether or 1,2-dimethoxyethane (glyme) were studied in order to investigate the structural and dynamic properties of ionic solutions in small and polymeric ethers. Full molecular dynamics simulations were performed at a range of different salt concentrations. An algorithm was designed which assigns ions to clusters and then calculates all the terms which contribute to ionic conductivity. In dilute solutions, free ions are the most common ionic species, followed by ion pairs. As the concentration increases, pairs become the most common species, with significant concentrations of clusters with 3 through 6 ions. Changing the solvent from dimethyl ether to glyme significantly decreases the ion clustering due to the chelate effect in which the two oxygens on a solvent stabilize an associated cation. The conductivity in stable systems is shown to be primarily the result of the movement of free ions and the relative movement of ions within neutral pairs. The Nernst-Einstein relation, commonly used in the discussion of polymer electrolytes, is shown to be inadequate to quantitatively describe conductivity in the model systems.