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

Excess properties of liquid mixtures from perturbation theory: results for model systems and predictions for real systems

Wetting transitions at the argon-solid-CO2 interface: Molecular-dynamics studies.

The calculation of chemical potentials from a WCA-type perturbation theory (PT) is described and two consistency tests are given. To check the results at infinite dilution of component A, Widom's test particle method is applied in combination with molecular dynamics (MD) simulations using vectorized codes on a CYBER 205. Technically, with increasing size of the test particles firstly the test particle cut-off radius and secondly the number of time steps has to be increased. Comparison of PT with MD results shows good agreement especially for σ AB /σ BB < 1 and e AB /e BB < 1 which is the parameter region for the solubility of gases (A) in liquids (B). The agreement is least satisfactory for decreasing σ AB /σ BB while simultaneously increasing e AB /e BB which is a typical ‘local composition’ effect for which PT does not account. Comparison with the results of Shing and Gubbins 1982, [Molec. Phys., 46, 1109] shows, however, discrepancies in some cases. Finally, predictions of Henry's constants for the mix...

Chemical potentials of model and real dense fluid mixtures from perturbation theory and simulations

The NpT + test-particle method for the calculation of vapor–liquid equilibria by molecular simulations is extended to ternary mixtures. It is applied to the system methane + ethane + carbon dioxide, for which all binary molecular interaction models are available from previous work. Methane is described as one-center Lennard-Jones fluid, ethane as two-center Lennard-Jones fluid, and carbon dioxide as two-center Lennard-Jones plus point quadrupole fluid. The unlike interactions are treated in the same way as the binary mixtures, using two parameters for each binary interaction. No ternary parameters are introduced. Vapor–liquid phase equilibria are calculated for the ternary mixture at the following temperature–pressure pairs: 233.15 K - 2 MPa; 250.5 K - 2 MPa; and 250.5 K - 3.04 MPa. Comparison of the simulation data with experimental and equation-of-state results shows excellent agreement. Bubble and dew densities are also reported.

Vapor–liquid equilibria of the ternary mixture CH4 + C2H6 + CO2 from molecular simulation

Vapour-liquid phase equilibria (VLE) are determined from the NpT plus test particle method for two-centre Lennard-Jones fluids of elongations L* = 0·22, 0·3292, 0·505, and 0·67. The resulting vapour pressures as well as the saturated vapour and liquid densities are correlated by simple equations. The thermodynamic consistency of the VLE data is confirmed on the basis of the Clausius-Clapeyron equation. Comparison is made with results from previous simulations, and from perturbation theory.

Johann Fischer

Papers

Excess properties of liquid mixtures from perturbation theory: results for model systems and predictions for real systems

Wetting transitions at the argon-solid-CO2 interface: Molecular-dynamics studies.

Chemical potentials of model and real dense fluid mixtures from perturbation theory and simulations

Vapor–liquid equilibria of the ternary mixture CH4 + C2H6 + CO2 from molecular simulation

Vapour-liquid equilibria of two-centre Lennard-Jones fluids from the NpT plus test particle method