Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The boundary layer equations for plane, incompressible, and steady flow are 

$$\matrix{ {u{{\partial u} \over {\partial x}} + v{{\partial u} \over {\partial y}} = - {1 \over \varrho }{{\partial p} \over {\partial x}} + v{{{\partial ^2}u} \over {\partial {y^2}}},} \cr {0 = {{\partial p} \over {\partial y}},} \cr {{{\partial u} \over {\partial x}} + {{\partial v} \over {\partial y}} = 0.} \cr }$$

Boundary Layer Theory

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

Numerical Initial Value Problems in Ordinary Differential Equations

New force fields for carbon dioxide and nitrogen are introduced that quantitatively reproduce the vapor–liquid equilibria (VLE) of the neat systems and their mixtures with alkanes. In addition to the usual VLE calculations for pure CO2 and N2, calculations of the binary mixtures with propane were used in the force-field development to achieve a good balance between dispersive and electrostatic (quadrupole–quadrupole) interactions. The transferability of the force fields was then assessed from calculations of the VLE for the binary mixtures with n-hexane, the binary mixture of CO2/N2, and the ternary mixture of CO2 /N2/propane. The VLE calculations were carried out using configurational-bias Monte Carlo simulations in either the grand canonical ensemble with histogram–reweighting or in the Gibbs ensemble.

Vapor–liquid equilibria of mixtures containing alkanes, carbon dioxide, and nitrogen

By using a simple, non-local version of the density functional theory, local densities are determined as functions of two variables: the distance from the pore wall and the distance along the pore axis. Attention is focused on evaluation of the local densities corresponding to ‘liquid–vapour’ equilibrium at the capillary condensation point. Specific calculations were carried out for a fluid interacting via a cut and shift Lennard-Jones potential between two parallel plates.

'Liquid-vapour' density profiles for fluids in pores from density functional theory

To optimize the processing of natural gas, accurate thermodynamic data are required. Here, we investigate the quality of the physically based BACKONE equation of state, which requires only 3−5 substance specific parameters for each pure component. For mixtures, only one mixture parameter for each binary and no ternary parameters are required. First, for 23 pure components (including ethylene, propylene, hydrogen sulfide, and water), the BACKONE parameters are given and the quality of the predictions for these 23 substances is described. Some characteristic results for properties such as vapor pressure, bubble and dew point density, as well as densities and caloric data, are presented. Second, for 21 key mixtures, the phase equilibria, densities, and caloric data are considered. Finally, we address liquefaction in three stages, considering vapor compression refrigeration cycles and isenthalpic expansion. By comparison with reference equations of state or experimental data, it turns out that, in all cases, ...

Accurate Thermodynamic Properties from the BACKONE Equation for the Processing of Natural Gas

The influence of unlike molecule interaction parameters on liquid mixture excess properties

Simulation studies on mixtures of dipolar with nonpolar linear molecules. II. A mixing rule for the dipolar contribution to the Helmholtz energy

Several results concerning the phase equilibria of two-centre Lennard-Jones (2CLJ) fluids are presented. Firstly, a comparison of chemical potential values for the elongation L = 0·63 obtained from perturbation theory shows qualitative agreement with the simulation results of Romano and Singer and excellent agreement with results of Guillot and Guissani except at the highest density. The complete saturation curve of the 2CLJ-0·63 fluid is then calculated by combining the Romano-Singer results, the Haar-Shenker-Kohler equation and perturbation theory. For the 2CLJ-fluids with L = 0·05, 0·10, and 0·20 the low-temperature saturation curves are calculated by perturbation theory as well as the pseudocritical points. At that point the packing fraction of the hard molecular cores has the constant value η = 0·165 in going from L = 0 to L = 0·3292 and falls off slightly for higher elongations. The reduced hard sphere diameter d/σ, however, shows a distinct minimum at about L = 0·27.

Johann Fischer

Papers

'Liquid-vapour' density profiles for fluids in pores from density functional theory

Accurate Thermodynamic Properties from the BACKONE Equation for the Processing of Natural Gas

The influence of unlike molecule interaction parameters on liquid mixture excess properties

Simulation studies on mixtures of dipolar with nonpolar linear molecules. II. A mixing rule for the dipolar contribution to the Helmholtz energy

Studies on phase equilibria of two-centre Lennard-Jones fluids