ON ALGORITHMS FOR BROWNIAN DYNAMICS COMPUTER SIMULATIONS

PoznańSeveral Brownian Dynamics numerical schemes for treating stochastic differential equations atthe position Langevin level are analyzed from the point of view of their algorithmic efficiency. The algorithmsare tested using model colloidal fluid of particles interacting via the Yukawa potential. Limitationsin the conventional Brownian Dynamics algorithm are shown and it is demonstrated that much betteraccuracy for dynamical and static quantities can be achieved with an algorithm based on the stochasticexpansion and second-order stochastic Runge-Kutta algorithms. Mutual merits of the second-order algorithmsare discussed

THE ALGORITHM FOR GENERATION OF CONNECTED CLOSED LINEAR GRAPHS FOR LATTICE-SPIN SYSTEMS

PoznańThe paper presents the algorithm and subroutine GRGEN for generation of connected closed linear graphs utilized in perturbation expansions. Starting with polygons and subsequently choosing pairs of not directly connected vertices in already generated graphs, the algorithm consequently projects one vertex onto the other one in the pair, generating only topologically different graphs. This algorithm is independent of the choice of a specific model and lattice geometry. The procedure is written in the FORTRAN 77 language, and is available at the Poznań Supercomputing and Networking Center on the SGI Power Challenge XL supercomputer

MOLECULAR DYNAMICS SIMULATIONS OF IONIC COPOLYMERS

PoznańMolecular Dynamics (MD) studies of ionic diblock copolymers are reported. The symmetrical diblocks are shown to exhibit a microphase separation transition (MST) similar to that of neutral diblocks. The MST was investigated by measuring thermodynamic, structural and dynamic properties: density, simulation box dimensions, structure factor, anisotropy in structure factor and directional diffusion constants. The slow cooling of the system from a high-temperature disordered phase to low-temperature ordered microphase separated phase was achieved in both temperature controlled ('NVT') MD, and temperature and pressure controlled ('NPT') MD

SIMULATIONS OF CRITICAL PROPERTIES OF CLASSICAL SPIN MODELS

PoznańTwo methods of simulations are described: an exact one - transfer matrixtechnique and a statistical one - Monte Carlo method. Both of them areapplied to investigate critical properties of classical spin models. To do thiswe also exploit finite size scaling and the critical point ratio of the squareof the second moment of the order parameter to its fourth moment. Generaldefinition of a classical spin model as well as particular definitions ofmodels are presented. Results of both methods are in good agreement and,moreover, they are consistent with numerical results provided by literature

THE STRUCTURE OF THE FIRST CO-ORDINATION SHELL OF Pb ATOMS IN LEAD-SILICATE GLASSES :A MOLECULAR DYNAMICS STUDY

PoznańThe structure of lead-silicate glasses, although being investigated for over sixty years, remains still controversial. One of the open questions is the structure of the lead subsystem: what are the basic structural units like, and how are they interconnected. In the literature the appearance of [PbO4]n, or [PbO3]n chains is reported even in the glasses of the same stoichiometry. In order to elucidate the problem of contradicting experimental findings, extensive molecular dynamics (MD) simulations in the isobaric-isoenthalpic (NpH) ensemble of the structure of lead-silicate glasses have been performed in the whole range of glassformation. The atoms were assumed to interact by a two-body Born-Mayer-Huggins interaction potential with full ionic charges. The potential parametrization was taken from Damodaran et al. [Phys. Chem. Glasses, 31, 212 (1990)1. The simulation box contained about 3000 atoms. For each composition an equilibrated ming Pb03 pyramids with the Pb atom in the vertices or in the middle of an al ost flat triangle formed from forapical oxygens of three different SiO4 tetrahedra. Another of all Pb atoms can be qualified as vertices ofrather distorted square pyramids. The remaining of lead atoms have neighbourhoods difficult to classify. For x 0.67 only a few Pb atoms were identified as remaining in the front of a face of a SiO4 tetrahedron. Most of the Pb atoms are placed in the vertices of edge sharing square PbO4 pyramids. Our MD results are compared with available experimental data. 0.1 X 0.33, about of all Pb atoms is either placed in the front of the faces of the SiO4 tetrahedra, forapical oxygens of three different SiO4 tetrahedra. Another of all Pb atoms can be qualified as vertices of rather distorted square pyramids. The remaining of lead atoms have neighbourhoods difficult to classify. For x 0.67 only a few Pb atoms were identified as remaining in the front of a face of a SiO4 tetrahedron

HIGH-PERFORMANCE COMPUTINGON HETEROGENEOUS SYSTEMS

PoznańA model two-processor heterogeneous computer consisting of one scalar and one vector processoris analyzed in terms of its performance. It is demonstrated that on mixed-type (scalar-vector) applicationsit is much more effective than a homogeneous environment. Various models of the job distribution areintroduced. A working implementation in the field of quantum chemistry is presented

SMOOTH PARTICLE APPLIED MECHANICS

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