Diffusion Monte Carlo: Exponential scaling of computational cost for large systems

The computational cost of a Monte Carlo algorithm can only be meaningfully discussed when taking into account the magnitude of the resulting statistical error. Aiming for a fixed error per particle, we study the scaling behavior of the diffusion Monte Carlo method for large quantum systems. We identify the correlation within the population of walkers as the dominant scaling factor for large systems. While this factor is negligible for small and medium sized systems that are typically studied, it ultimately shows exponential scaling. The scaling factor can be estimated straightforwardly for each specific system and we find that is typically only becomes relevant for systems containing more than several hundred atoms.Comment: 6 pages, 3 figures, published by Phys. Rev. B (further changes following referee's reports

Coccinia intermedia

Nuclear and plastid sequences from two individuals of a suspected new species of Coccinia from West Africa were added to an available molecular phylogeny for the remaining 27 species of the genus. Phylogenetic analyses of these data indicate the new species' monophyletic status and closest relatives. Based on four fertile collections, we here describe and illustrate Coccinia intermedia Holstein. We also provide a key to the Coccinia species of West Africa and map their distributions

Ion-by-Ion DEM Determination: I. Method

We describe a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. This technique involves fitting spectra using a number of components, each of which is the entire X-ray line emission spectrum for a single ion. It is applicable to high-resolution X-ray spectra of any collisionally ionized plasma and particularly useful for spectra in which the emission lines are broadened and blended such as those of the winds of hot stars. This method does not require that any explicit assumptions about the form of the DEM distribution be made and is easily automated.Comment: This paper was split in two. This version is part I. Part II may be found at astro-ph/050343

AGMAAS: a GIS integrated tool for modelling wind-borne spreading of FMD virus

The aim of our work was to develop a tool integrated into Quantum GIS (QGIS) that can help
the user estimate and visualize the possible infective areas around an outbreak based on HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model

High Temperature Expansion for Frustrated and Unfrustrated S=1/2 Spin Chains

A computer aided high temperature expansion of the magnetic susceptibility and the magnetic specific heat is presented and demonstrated for frustrated and unfrustrated spin chains. The results are analytic in nature since the calculations are performed in the integer domain. They are provided in the form of polynomials allowing quick and easy fits. Various representations of the results are discussed. Combining high temperature expansion coefficients and dispersion data yields very good agreement already in low order of the expansion which makes this approach very promising for the application to other problems, for instance in higher dimensions.Comment: 13 pages, 8 figures, to appear in Eur. Phys. J. B, minor corrections, correction of a[5] in table A.1.a, discussion of the region of validity added, coefficients available electronically: http://www.thp.uni-koeln.de/~g