The static dipole polarizabilities of helium and molecular hydrogen by differential diffusion Monte Carlo

The differential diffusion Monte Carlo method, involving correlated random walks, is used to calculate the static polarizabilities of molecular hydrogen and helium by application of a finite electrostatic field. The results are for molecular hydrogen (alpha)=4.60(3) au; (alpha)|=6.38(5) au; for helium (alpha) = 1.38(1) au. The results agree, within the statistical errors, with those obtained by application of high quality ab initio methods

Root-Mean-Square Dipole Moment and Neutron Scattering Function of 18-Crown-6 in Cyclohexane: Comparison of Three Potential Models

Molecular dynamics simulations have been performed in order to study the effect of a new charge density distribution for 18-crown-6, recently described in the literature [J. Mol. Struct. (THEOCHEM) 1994, 305, 2491, on different properties of the crown ether. Comparisons are made with results discussed in a previous paper where other potential models were employed. Remarkable shifts in the spectrum of conformations are observed, and for the first time an average dipole moment is calculated that is in good agreement with experiment. We have calculated neutron scattering cross sections and X-ray intensities for two potential models. Of these, only the neutron scattering cross section turned out to be sensitive to the differences in the structures resulting from these potentials

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

Critical Analysis of Non-Nuclear Electron-Density Maxima and the Maximum Entropy Method

Experimental evidence for the existence of non-nuclear maxima in charge densities is questioned. It is shown that the non-nuclear maxima reported for silicon are artifacts of the maximum entropy method that was used to analyze the x-ray diffraction data. This method can be improved by the use of appropriate prior information. We report systematic tests of the improved method leading to the absence of non-nuclear maxima in Si. Likewise, the non-nuclear maxima reported earlier in beryllium are not substantiated.\ud \u

Domain formation and growth in spinodal decomposition of a binary fluid by molecular dynamics simulations

The two initial stages of spinodal decomposition of a symmetric binary Lennard-Jones fluid have been simulated by molecular dynamics simulations, using a hydrodynamics-conserving thermostat. By analyzing the growth of the average domain size R(t) with time, a satisfactory agreement is found with the R(t)t1/3 Lifshitz-Slyozov growth law for the early diffusion-driven stage of domain formation in a quenched homogeneous mixture. In the subsequent stage of viscous-dominated growth, the mean domain size appears to follow the linear growth law predicted by Siggia

Molecular dynamics of 18-crown-6 complexes with alkali-metal cations and urea: Prediction of their conformations and comparison with data from the cambridge structural database

Complexes of 18-crown-6 with alkali-metal cations (Na+, K+, and Rb+), urea, and the uncomplexed crown ether were studied in vacuo with the molecular dynamics method. Conformational data from these calculations (simulation times in the range from 6-15 ns) was compared with information from the Cambridge Structural Database. Despite the differences in condition between the simulations and the solid state, a number of interesting similarities are observed

Discrete particle simulation of bubble and slug formation in a two-dimensional gas-fluidised bed: a hard-sphere approach.

A discrete particle model of a gas-fluidised bed has been developed and in this the two-dimensional motion of the individual, spherical particles was directly calculated from the forces acting on them, accounting for the interaction between the particles and the interstitial gas phase. Our collision model is based on conservation laws for linear and angular momentum and requires, apart from geometrical factors, two empirical parameters: a restitution coefficient and a friction coefficient. A sequence of collisions is processed using techniques which find their application in hard-sphere simulations which are commonly encountered in the field of molecular dynamics. The hydrodynamic model of the gas phase is based on the volume-averaged Navier-Stokes equations. Simulations of bubble and slug formation in a small two-dimensional bed (height 0.50 m, width 0.15 m) with 2400 particles (dp = 4 mm, material: aluminium, p = 2700 kg m¿3) showed a strong dependency of the flow behaviour with respect to the restitution and friction coefficient. A preliminary experimental validation of our model was performed using a small scale "two-dimensional" gas-fluidised bed (height 0.30 m, width 0.15 m, depth 0.015 m) with 850 ¿m ballotini glass particles (p = 2930 kg m¿3) as the bed material. Results compared fairly well with the results of a simulation which was performed with 40,000 particles using realistic values for the restitution and friction coefficients which were obtained from simple independent experiment

Coarse graining of slow variables in dynamic simulations of soft matter

A new Brownian dynamics model is presented to describe the coarse grain dynamics of particles with long-lived memory. Instead of solving a set of generalized Langevin equations we introduce a set of variables describing the slowly fluctuating thermodynamic state of the ignored degrees of freedom. These variables give rise to additional transient forces on the simulated particles, whose interpretation provides a new way of thinking about memory effects in soft-matter physics. We illustrate the proposed method by simulating shear thinning of synthetic resins.\u