Determining Free Energy Differences Through Variational Morphing

Free energy calculations based on atomistic Hamiltonians and sampling are key to a first principles understanding of biomolecular processes, material properties, and macromolecular chemistry. Here, we generalize the Free Energy Perturbation method and derive non-linear Hamiltonian transformation sequences for optimal sampling accuracy that differ markedly from established linear transformations. We show that our sequences are also optimal for the Bennett Acceptance Ratio (BAR) method, and our unifying framework generalizes BAR to small sampling sizes and non-Gaussian error distributions. Simulations on a Lennard-Jones gas show that an order of magnitude less sampling is required compared to established methods.Comment: 7 pages, 5 figure

Mixing effectiveness in the Apollo oxygen tanks of spin-up and rotation-reversal maneuvers

Two-dimensional simulations of stratified flows in the Apollo oxygen tanks have been used to estimate the mixing effectiveness of spin-up and rotation-reversal maneuvers. Calculations have been made for square and circular cylindrical tank geometries. Differences arising from heater position on the tank wall or near the center of the tank have been investigated. In the event of a prolonged period without normal maneuvers, the potential pressure decay (drop in pressure that would result from adiabatic mixing) can be suppressed by more than a factor of two through the use of spin-up and rotation-reversal maneuvers. Changes in rotation rate of order three revolutions per hour or greater are sufficient for this purpose

Flexibilisierung von Arbeitszeiten und Arbeitsformen in Bibliotheken

Die vorliegende, von Studierenden der Fachhochschule für Bibliothekswesen in Frankfurt am Main erstellte Projektarbeit befaßt sich mit Fragen, Aspekten und Möglichkeiten der Flexibilisierung von Arbeitszeiten und Arbeitsformen in Bibliotheken. Im ersten Teil der Arbeit werden die wichtigsten Arbeitsteilzeitmodelle vorgestellt und deren Vorteile für Arbeitgeber und nehmer herausgearbeitet. Anschließend erfolgt eine durch Graphiken und Tabellen veranschaulichte Untersuchung von drei ausgewählte Universitätsbibliotheken (Gießen, Marburg, Mainz) in bezug auf bereits existierende Arbeitszeitflexibilisierung. Ein Ausblick geht der Frage nach, ob die dargestellten Teilzeitmodelle für Bibliotheken notwendig und ausreichend sind. Der zweite Themenkomplex befaßt sich mit der Telearbeit als einer innovativen Arbeitsform in Bibliotheken. Ausgehend von einer Begriffsbestimung werden verschiedene Formen der Telearbeit sowie deren Vor und Nachteile geschildert. Es wird erläutert, welche Bedingungen an den Telearbeiter und seinen Arbeitsplatz geknüpft sind, welche rechtlichen Voraussetzungen erfüllt werden müssen, welche finanziellen Aspekte eine Rolle spielen und welche Anwendungsmöglichkeiten für Telearbeit in Bibliotheken bestehen. An drei konkreten Beispielen wird dann geschildert, wie Telearbeit in Bibliotheken bereits verwirklicht wurde. Abschließend wird der Frage nachgegangen, welche Perspektiven der Einsatz von Telearbeit in Bibliotheken hat. Der dritte Abschnitt der Ausarbeitung stellt das bibliothekarische Call Center als neue Form einer benutzerorientierten Einrichtung vor. Zielsetzungen und Dienstleistungsformen des Call Centers werden ebenso abgehandelt wie Aspekte der Planung, Einrichtung und Arbeitsplatzergonomie sowie des Einsatzes von Telearbeit. Anhand des Beispiels der Zentral und Landesbibliothek Berlin wird schließlich dokumentiert, wie sich ein bibliothekarisches Call Center in der Praxis bereits bewährt hat

Phyllotaxis involves auxin drainage through leaf primordia

The spatial arrangement of leaves and flowers around the stem, known as phyllotaxis, is controlled by an auxin-dependent reiterative mechanism that leads to regular spacing of the organs and thereby to remarkably precise phyllotactic patterns. The mechanism is based on the active cellular transport of the phytohormone auxin by cellular influx and efflux carriers, such as AUX1 and PIN1. Their important role in phyllotaxis is evident from mutant phenotypes, but their exact roles in space and time are difficult to address due to the strong pleiotropic phenotypes of most mutants in phyllotaxis. Models of phyllotaxis invoke the accumulation of auxin at leaf initials and removal of auxin through their developing vascular strand, the midvein. We have developed a precise microsurgical tool to ablate the midvein at high spatial and temporal resolution in order to test its function in leaf formation and phyllotaxis. Using amplified femtosecond laser pulses, we ablated the internal tissues in young leaf primordia of tomato (Solanum lycopersicum) without damaging the overlying L1 and L2 layers. Our results show that ablation of the future midvein leads to a transient accumulation of auxin in the primordia and to an increase in their width. Phyllotaxis was transiently affected after midvein ablations, but readjusted after two plastochrons. These results indicate that the developing midvein is involved in the basipetal transport of auxin through young primordia, which contributes to phyllotactic spacing and stability

Nucleon Structure Functions from Relativistic Constituent Quarks

We investigate deep inelastic lepton scattering from the nucleon within a constituent quark picture, in which the internal structure of constituent quarks is modeled by meson and diquark dressing. In a covariant framework this structure leads to a breakdown of the factorization necessary for a convolution formulation. We perform our analysis in time-ordered perturbation theory in the infinite momentum frame where factorization of subprocesses is automatic. Numerical results are compared with recent data on valence quark distributions in the proton.Comment: 10 pages LaTeX, 3 figures available upon request, TPR-94-7, to appear in Physics Letters

Confinement at Weak Coupling

The free energy of U(N) and SU(N) gauge theory was recently found to be of order N^0 to all orders of a perturbative expansion about a center-symmetric orbit of vanishing curvature. Here I consider extended models for which this expansion is perturbatively stable. The extreme case of an SU(2) gauge theory whose configuration space is restricted to center-symmetric orbits has recently been investigated on the lattice hep-lat/0509156. In extension of my talk, a discussion and possible interpretation of the observed finite temperature phase transition is given. The transfer matrix of constrained SU(N) lattice gauge theory is constructed for any finite temperature.Comment: 8 pages, no figures, updated talk given at LC2005 in Cairns, Australi