S.W. de Leeuw

An Iterative PPPM Method for Simulating Coulombic Systems on Distributed Memory Parallel Computers

Abstract We describe results obtained from a new implementation of Hockneys Particle-Particle Particle-Mesh (PPPM) method for evaluation of Coulomb energies and forces in simulations of charged particles. Rather than taking the usual approach, solving Poissons equation by means of a Fourier transformation, we use an iterative Poisson solver. In a molecular dynamics (MD) simulation the solution from the previous time-step provides a good starting point for the next solution. This reduces the number of iterations per time-step to acceptable values. The iterative scheme has a complexity O(N), and, in contrast with the Fourier transform based approach, it is easily implemented on a parallel architecture with a minimum of communication overhead. We examine the origin of the errors in the algorithm and find that reasonable accuracies in the Coulomb interaction can best be attained by making the charge density profile as smooth as possible. This involves spreading the particle charges over a large number of gr...

Molecular dynamics study of the thermodynamic and structural properties of methanol and polarizable/non‐polarizable carbon tetrachloride mixtures

Molecular dynamics simulations have been performed for methanol/carbon tetrachloride mixtures over the whole composition range at 323 K and zero pressure. The OPLS (optimized potentials for liquid simulation) potential energy parameters by Jorgensen were used to model the methanol potential. Both a non‐polarizable carbon tetrachloride model taken from McDonald, Bounds, and Klein [Mol. Phys. 45, 521 (1982)] as well as a polarizable model were used. The latter model was devised by combining the model of McDonald, Bounds, and Klein with the atomic polarizabilities proposed by Applequist, Carl, and Fung [J. Am. Chem. Soc. 94, 2952 (1972)]. We show that the role of the methanol–carbon tetrachloride interactions are very important in discussing the thermodynamic mixing properties. In order to reproduce the asymmetric behavior of the excess enthalpies with respect to composition it is necessary to include the non‐additive polarization interaction. The structure and especially hydrogen bonding properties are disc...

Molecular dynamics studies of polar/nonpolar fluid mixtures. I. Mixtures of Lennard‐Jones and Stockmayer fluids

The results of canonical ensemble molecular dynamics calculations of mixtures of Lennard–Jones and Stockmayer fluids are reported. To study solely the influence of the polarity, the Lennard‐Jones parameters were identical for both components. The excess mixing properties show a strong asymmetry with respect to composition for large dipolar strength. The free energy of mixing is obtained through a thermodynamic integration procedure. The results strongly suggest that, for reduced dipolar strengths μ2>3.15, demixing occurs into a phase rich in polar component and an almost pure Lennard‐Jones fluid. It is shown that perturbation theory yields fairly accurate results for the dipolar energy and free energy of the mixture. For the free energy of mixing, qualitatively correct results are obtained. The structure and orientational correlation functions of the mixture are discussed. The radial distribution function for pairs of polar molecules show a marked increase in local ordering with dipolar strength for low c...

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.

Computer simulation of ionic systems. Influence of boundary conditions

The results of molecular dynamics calculations for systems of charged particles under periodic boundary conditions are reported for the case in which the periodic array of particles makes a macroscopically large sphere surrounded by a continuum of dielectric constant ϵ′. It is shown that thermodynamic and most dynamic properties are independent of the nature of the surrounding medium. The conductivity σ(ω) of the system depends strongly on the dielectric properties of the surrounding medium.

The dynamics in polyethyleneoxide–alkali iodide complexes investigated by neutron spin-echo spectroscopy and molecular dynamics simulations

We determined the self part of the intermediate scattering function in liquid polyethyleneoxide (PEO) and PEO–alkali iodide complexes by means of neutron spin-echo spectroscopy and molecular dynamics (MD) computer simulations. We present the first accurate quantitative results on the segmental dynamics in the time range up to 1 ns and the wave-vector range from a few nm?1 to approximately 20 nm?1. We investigate the influence of polymer chain length, salt concentration, and cation type. We find that the neutron data and MD data for pure PEO agree very well. A relatively small concentration of dissolved salt (1 metal ion per 15 monomers) leads to a slowing down of the segmental motions by an order of magnitude. Here, the MD simulations agree qualitatively. Increasing the chain length from 23 to 182 monomers has no significant effect except at the highest salt concentration. Similarly, changing the cation from Li to Na hardly makes any difference. The Rouse model does not adequately describe our data.

Molecular dynamics simulation of nanoporous silica

The micro-structure and dynamical properties of nanoporous silica are investigated using the molecular dynamics simulation method. The porous silica obtained have densities down to 0.7 g/ml and a mass fractal dimension of 2.6 or higher. The specific surface of the porous silica is calculated and found to be in agreement with adsorption experiments. The vibrational and dielectric power spectra show an enhanced intensity in the low frequency region. The low frequency modes are characterised by a covariance matrix eigenvector analysis and found to correspond to cluster motion.

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.

Ionic conduction in Na+-β-alumina studied by molecular dynamics simulation

Abstract The structure and dynamics of the fast-ionic conductor Na+-β-alumina are studied by classical molecular dynamics simulation. Four different compounds are simulated: stoichiometric-, non-stoichiometric- and two Mg-stabilised β-aluminas. The Beevers–Ross (BR)-site is the most stable position for stoichiometric Na+ ions in the conduction plane. Excess Na+ ions are found to occupy positions that are slightly off-center from the aBR lattice site. This shifted aBR, or A-site, occupation is associated with a reconstruction of three ions and a neighbouring BR vacancy. Interstitial oxygens in the conduction plane stabilise the A-site by creating permanent BR vacancies. The diffusion coefficients and conductivities as a function of temperature display close to Arrhenius behaviour. Results are in agreement with experiment for the 300–900 K regime. We find a tendency for an increasing apparent activation energy at higher temperatures.

Vibrational spectra of rings in vitreous silica

Abstract We compute the local density-of-states associated with the coherent oxygen breathing motion of three and four membered rings in vitreous silica. While some individual rings have sharp responses with widths as small as 8 cm -1 , the system as a whole will not have as sharp a response as observed experimentally in the D 1 and D 2 “defect lines” unless the rings are very homogeneous in shape .