Catálogo de publicaciones - libros

Using a parallelized density matrix renormalization group (DMRG) code we demonstrate the potential of the DMRG method by calculating ground-state properties of two-dimensional Hubbard models. For 7 × 6, 11 × 6 and 14 × 6 Hubbard ladders with doped holes and cylindrical boundary conditions (BC), open in -direction and periodic in the 6-leg -direction, we comment on recent conjectures about the appearance of stripe-like features in the hole and spin densities. In addition we present results for the half-filled 4 ×4 system with periodic BC, advance to the 6 × 6 case and pinpoint the limits of the current approach.

We present equilibrium geometries, dipole moments, ionization energies and electron affinities of the DNA base molecules adenine, thymine, guanine, and cytosine calculated from . The comparison of our results with experimental data and results obtained by using quantum chemistry methods shows that gradient-corrected density-functional theory (DFT-GGA) calculations using ultra-soft pseudopotentials and a plane-wave basis are a numerically efficient and accurate alternative to methods employing localized orbitals for the expansion of the electron wave functions.

Direct numerical simulations (DNS) of compressible supersonic channel flow of air at Reynolds numbers ranging from = 180 to = 560 and Mach numbers ranging from = 0.3 to = 3.0 have been performed. A Navier-Stokes solver of high order accuracy has been vectorized and parallelized to run efficiently on the Hitachi SR8000-F1. Budgets of the Reynolds stresses and the passive scalar fluxes are presented, as well as explanations concerning the reduction of the pressure-correlation terms, using a Green's function approach.

We discuss performance characteristics of scientific applications on modern computer architectures, ranging from commodity “off-the-shelf” (COTS) systems like clusters, to tailored High Performance Computing (HPC) systems, e.g. NEC SX6 or CRAY X1. The application programs are selected from important HPC projects which have been supported by the KONWIHR project cxHPC. In general we focus on the single processor performance and give some optimisation/parallelisation hints, if appropriate. For computational fluid dynamics (CFD) applications we also discuss parallel performance to compare COTS with tailored HPC systems. We find, that an HPC environment with a few tailored “central” high-end systems and “local” mid-size COTS systems supports our users' requirements best.

In deformations of polynomial functions one may encounter “singularity exchange at infinity” when singular points disappear from the space and produce “virtual” singularities which have an influence on the topology of the limit polynomial. We find several rules of this exchange phenomenon, in which the total quantity of singularity turns out to be not conserved in general.

We report on the successful numerical implementation of an original method for the accurate quantum treatment of helium under electromagnetic driving. Our approach is the first to allow for a description of the highly complex quantum dynamics of this system, in the entire non-relativistic parameter regime, i.e., it provides full spectral and dynamical information on the ionization of the atomic ground state by optical fields, as well as on the dynamics of doubly excited Rydberg states under radiofrequency driving. As a by-product, the non-trivial role of the dimension of configuration space for the field-free dynamics of doubly excited helium is elucidated.

We report on a consistent, practically converged microscopic calculation of the scattering states in the He system employing modern realistic two-nucleon and three-nucleon potentials in the framework of the resonating group model (RGM). Comparisons are made for selected examples of phase shifts and data.

We describe our recently initiated project for the non-perturbative study of heavy quark systems in quenched lattice QCD. Motivated by the desire to avoid additional approximations, we work on fine lattices which are large in terms of the number of lattice points. The physical quantities which we want to compute are discussed, as well as the prospects for studying their dependence on the mass of the heavy quark.

Quantum-Chromodynamics (QCD) is the theory of quarks, gluons and their interaction. It has an important almost exact symmetry, the so-called chiral symmetry (which is actually broken spontaneously). This symmetry plays a major role in all low-energy hadronic processes. For traditional formulations of lattice QCD, CPU-time and memory limitations prevent simulations with light quarks and this symmetry is seriously violated. During the last years successful implementations of the chiral symmetry for lattice QCD have been constructed. We use two approximate implementations (both of them in the quenched approximation) with different specific advantages. We have also made progress towards the development of a practical algorithm to allow for simulations with dynamical quarks. In 2003 a series of discoveries of a new class of particles, called pentaquarks, has created very strong interest in lattice studies of resonance states. We have performed such studies with a specific method for the N* resonances with very satisfying results and are currently working on similar calculations for the pentaquarks. We have also addressed the question, which type of gauge field configurations is responsible for confinement and chiral symmetry breaking. Finally we are calculating three-point functions. We hope that for the small quark masses which we reach the results will not only be of direct phenomenological interest, but will also test predictions from chiral perturbation theory.

Non-linear highly energetic plasma phenomena play a key-role in the understanding of astrophysical objects. We present plasma scenarios that provide a valid description for coherent radiation emission features observed from pulsar magnetospheres and for the self-consistent magnetic field generation essential for -ray burst synchrotron models. For this purpose we study ultra-relativistic plasma shell collisions with ensembles of up to 10 particles. We introduce the framework of the Particle-In-Cell (PIC) approach, details of the numerical realization and performance issues on the IBM REGATTA system of the Rechenzentrum Garching and the HITACHI supercomputer of the Leibniz Rechenzentrum. A final overview on simulation results and future perspectives closes the contribution.