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

Working fluids for high-temperature organic Rankine cycles

Using an improved version of the recently suggested NpT + test particle method, phase equilibria are determined for the Lennard-Jones fluid in the temperature range T* = 0·70 to T* = 1·30 in steps of ΔT* = 0·05. For the final results, gas simulations are necessary only at the two highest temperatures; for T* = 1·20 and downwards the gas phase can be described with sufficient accuracy by the Haar-Shenker-Kohler equation. The resulting vapour pressures, bubble and dew densities can be correlated by the equations , , , with the critical temperature and density being T*c = 1·310 and ρ*c = 0·314, respectively. Then a detailed comparison with other simulation results and perturbation theory results is given. Outstanding findings are that the present data are rather smooth, that they match quite well the early data of Hansen and Verlet (1969, Phys. Rev., 184, 151) and are much closer to perturbation theory than most previous results. Finally, using the vapour pressure equation and directly calculated volume chan...

Vapour liquid equilibria of the Lennard-Jones fluid from the NpT plus test particle method

In this work we present new molecular dynamics simulation results for the liquid–vapor interface of the pure Lennard-Jones fluid. Our aims were further investigations on the simulation setup and the simulation parameters to obtain reliable data for the coexisting densities as well as for the surface tension. The influence of the cut-off distance to the interfacial properties is investigated and long-range corrections to both the dynamics and the surface tension are applied. It is found that the saturated liquid densities from the surface simulations agree with those from the NpT+test particle method within 1% for sufficiently large simulation boxes; the saturated vapor densities agree within 4%. In order to obtain reliable values for the surface tension, cut-off radii of at least 5 molecular diameters supplemented by a tail correction are required.

Molecular dynamics simulation of the liquid–vapor interface: The Lennard-Jones fluid

Comparison of trilateral cycles and organic Rankine cycles

Model fluids consisting of spherical n/6 molecules and of linear two-centre Lennard-Jones molecules are considered. The orthobaric densities and the vapour pressures are calculated by using perturbation theory at liquid densities and second virial coefficients at gas densities. In order to classify the results the deviations from corresponding states principle are investigated. For spherical molecules, a steeper repulsive potential lowers the vapour pressure considerably but hardly effects the liquid saturation density. For linear molecules, an increasing elongation lowers the vapour pressure and increases the liquid saturation density. Finally, comparison with real Ar, CH4, N2, F2, C2H6, and CO2 is made.

Johann Fischer

Papers

Working fluids for high-temperature organic Rankine cycles

Vapour liquid equilibria of the Lennard-Jones fluid from the NpT plus test particle method

Molecular dynamics simulation of the liquid–vapor interface: The Lennard-Jones fluid

Comparison of trilateral cycles and organic Rankine cycles

Influence of intermolecular potential parameters on orthobaric properties of fluids consisting of spherical and linear molecules