Catálogo de publicaciones - libros

Reactive scattering is one of the fundamental processes in atomic and molecular collision dynamics. Reactions of hydrogen systems are of a particular interest in this respect, because they are amenable to the most rigorous theoretical treatment and thus represent ideal prototype cases for a detailed comparison of theory and experiment. This is best exemplified by the neutral hydrogen system H + H and F + H or the ionic system He + H. Quantum chemistry has provided a very accurate potential energy surface for these systems, especially for H + H. The collision dynamics is treated by quasi-classical trajectory or by rigorous, fully converged quantum methods. There is a considerable number of very detailed experimental results on which theory can be tested.

We present ab initio molecular dynamics (MD) simulations of the simplest amino acid, glycine, at the water / pyrite interface under extreme pressure / temperature conditions. These simulations are aimed to contribute to the discussion of the “iron-sulfur world” (ISW) scenario, an intriguing proposal in the controversial field of “Origin of Life” research. The simulations show that glycine easily desorbs from a water / pyrite interface through hydrogen-bond assistance. The retention time is only of the order of a picosecond and the surface bonding is best understood as a relatively weak electrostatic interaction. However, we have found indications of glycine activation due to the interaction with the surface, and thus for a possible reaction with a suitable anchor molecule.

Two papers in this volume deal with more theoretical aspects of computer sci-ence related to supercomputing. Both focus on the problem of benchmarking for high performance computer systems. This emphasises both the impor-tant role and the extreme difficulty of finding adequate methods to evaluate the performance of high speed systems. Both papers deal with performance aspects of parallel programs.

As the successor of the project was started to find some answers to questions the first project left open. The main problem encountered in examining SX-4 and SX-5 at the was the impossibility to test these machines with all planed trials concerning mixed workloads due to software hangups. To illustrate the problem as well as the undertaken efforts to solve it, we will divide this report into three parts. First, the used benchmark system is described briefly. Thereafter, the results and problems investigating the SX vector computers are shown. In the final part the changes made to as well as the new findings are discussed.

SKaMPI is now an established benchmark for MPI implementations. The development of SKaMPI-5 strives for improvements in several directions: (i) extension of the benchmark to cover more functionality of MPI, (ii) construction of a collection of collective algorithm kernels which are not supported by core MPI collective operations. (iii) a redesign of the SKaMPI benchmark allowing it to be extended more easily (thus matching requests from SKaMPI users).

In the present paper we give an overview of the extension of SKaMPI for the evaluation of virtual topologies, describe the foundations of new algorithms for fast all-to-all communication specifically tailored for the case of differing message sizes, and give a first impression of what SKaMPI-5 will look like, for which we now have a prototype running.

Earth sciences belong to those scientific disciphnes which have to rely on high-performance computing not only because of the complexity of the prob-lems this discipline has to cope with but also for other reasons. Since the geometric structures are in general three-dimensional, the storage capacities required are large, and because of the large amount of data to be processed, high computing speeds are also required. For the second time the transac-tions contain two articles on this subject in this chapter. They clearly confirm that application of high-performance computing in analyses of the earth sci-ences is an absolute must. Needless to say, that the articles reported here are concerned with key problems of the subject.

This paper is concerned with numerical considerations of fluid effects on wave propagation. The focus is on effective elastic properties (i.e. velocities) in different kinds of dry and fluid-saturated fractured media. We apply the so-called rotated staggered finite-difference grid (RSG) technique. Using this modified grid it is possible to simulate the propagation of elastic waves in a 2D or 3D medium containing cracks, pores or free surfaces without explicit boundary conditions and without averaging elastic moduli. Therefore the RSG allows an efficient and precise numerical study of effective velocities in fractured structures. This is also true for structures where theoretically it is only possible to predict upper and lower bounds. We simulate the propagation of plane P- and S-waves through three kinds of randomly cracked 3D media. Each model realization differs in the porosity of the medium and is performed for dry and fluid-saturated pores. The synthetic results are compared with the predictions of the well known Gassmann equation and the Biot velocity relations. Although we have a very low porosity in our models, the numerical calculations showed that the Gassmann equation cannot be applied for isolated pores (thin penny-shaped cracks). For Fontainebleau sandstone we observe with our dynamic finite-difference approach the exact same elastic properties as with a static finite-element approach. For this case the Gassmann equation can be checked successfully. Additionally, we show that so-called open-cell Gaussian random field models are an useful tool to study wave propagation in fluid-saturated fractured media. For all synthetic models considered in this study the high-frequency limit of the Biot velocity relations is very close to the predictions of the Gassmann equation. However, using synthetic rock models saturated with artificial “heavy” water we can roughly estimate the corresponding tortuosity parameter.

This is a report on first steps for a combination of two numerical models of the evolution of the Earth’s mantle: The first one, K3, is a new 2-D convection-fractionation model that simulates the growth of continents and of the geochemically complementary depleted mantle reservoir. The second model shows the 3-D generation of oceanic lithospheric plates and subducting sheet-like downwellings in a spherical-shell mantle. Based on the abundances of the present-day geochemical reservoirs of Hofmann (1988) we developed a numerical dynamical model of convection and of chemical differentiation in the Earth’s mantle. It is shown that a growing and additionally laterally moving continent and a growing depleted mantle evolved from an initially homogeneous primordial mantle. The internal heat production density of the evolving mantle depends on the redistribution of the radioactive elements by fractionation and convection. The fractionation generates separate geochemical reservoirs. However, the convection blurs the reservoirs by mixing. Although we take into account also the effects of the two phase transitions in 410 and 660 km depth, it is essentially the dependence of the viscosity on radius which guarantees the conservation of the major geochemical reservoirs. This model has no internal compulsory conditions. The principal idea of this first model is to compute the relative viscosity variations as a function of depth from observable quantities. We develop a self-consistent theory using the Helmholtz free energy, the Ullmann-Pan’kov equation of state, the free volume Grüneisen parameter and Gilvarry’s formulation of Lindemann’s law. In order to receive the relative variations of the radial factor of the viscosity, we insert the pressure, , the bulk modulus, , and from PREM. For mantle layers deeper than 771 km we used the perovskite melting curve by Zerr and Boehler (1993, 1994) in order to estimate the relative viscosity. For the calibration of the viscosity we have chosen the standard postglacial-uplift viscosity beneath the continental lithosphere. Furthermore, we took into account the dependence of the viscosity on temperature and on the degree of depletion of volatiles. An essential first new result of this paper is a and a layer below it. Although our model mantle is essentially heated from within, we assume additionally a small heat flow at the CMB. This is necessary because of the dynamo theory of the outer core. The second main result of this first model is a in a depleted upper part and a lower part rich in incompatible elements, yet. This result is rather insensitive to variations of the Rayleigh number and of the thermal boundary condition at CMB. The different parts of this paper are closely connected by the algorithm. The continuation of the first finding leads to a 3-D, up to now purely thermal model of mantle evolution and plate generation. This second model was used to carry out a series of three-dimensional compressible spherical-shell convection calculations with another new, but related viscosity profile, called , that is derived from PREM and mineral physics, only. Here, the Birch-Murnaghan equation was used to derive the Grüneisen parameter as a function of depth. Adding the pressure dependence of the thermal expansion coefficient of mantle minerals, we derived the specific heats, and , too. Using the Gilvarry formulation, we found a new melting temperature of the mantle and the new viscosity profile, . The features of are a high-viscosity transition layer, a second low-viscosity layer beginning under the 660-km discontinuity, and a strong viscosity maximum in the central parts of the lower mantle. The rheology is Newtonian but it is supplemented by a viscoplastic yield stress, . A viscosity-level parameter, , and have been varied. For a medium-sized Rayleigh-number-yield-stress area, generates a stable, plate-tectonic behavior near the surface and simultaneously thin sheet-like downwellings in the depth. Outside this area three other types of solution were found. The presence of internal low-viscosity layers and of ∂ is obviously conducive for plateness and thin sheet-like downwellings. The distribution of the downwellings is more Earth-like if the yield stress is added. The outlines of a combination of the two models have been discussed.