Langevin Thermostat for Rigid Body Dynamics

We present a new method for isothermal rigid body simulations using the quaternion representation and Langevin dynamics. It can be combined with the traditional Langevin or gradient (Brownian) dynamics for the translational degrees of freedom to correctly sample the NVT distribution in a simulation of rigid molecules. We propose simple, quasi-symplectic second-order numerical integrators and test their performance on the TIP4P model of water. We also investigate the optimal choice of thermostat parameters.Comment: 15 pages, 13 figures, 1 tabl

Space tug propulsion system failure mode, effects and criticality analysis

For purposes of the study, the propulsion system was considered as consisting of the following: (1) main engine system, (2) auxiliary propulsion system, (3) pneumatic system, (4) hydrogen feed, fill, drain and vent system, (5) oxygen feed, fill, drain and vent system, and (6) helium reentry purge system. Each component was critically examined to identify possible failure modes and the subsequent effect on mission success. Each space tug mission consists of three phases: launch to separation from shuttle, separation to redocking, and redocking to landing. The analysis considered the results of failure of a component during each phase of the mission. After the failure modes of each component were tabulated, those components whose failure would result in possible or certain loss of mission or inability to return the Tug to ground were identified as critical components and a criticality number determined for each. The criticality number of a component denotes the number of mission failures in one million missions due to the loss of that component. A total of 68 components were identified as critical with criticality numbers ranging from 1 to 2990

New Langevin and Gradient Thermostats for Rigid Body Dynamics

We introduce two new thermostats, one of Langevin type and one of gradient (Brownian) type, for rigid body dynamics. We formulate rotation using the quaternion representation of angular coordinates; both thermostats preserve the unit length of quaternions. The Langevin thermostat also ensures that the conjugate angular momenta stay within the tangent space of the quaternion coordinates, as required by the Hamiltonian dynamics of rigid bodies. We have constructed three geometric numerical integrators for the Langevin thermostat and one for the gradient thermostat. The numerical integrators reflect key properties of the thermostats themselves. Namely, they all preserve the unit length of quaternions, automatically, without the need of a projection onto the unit sphere. The Langevin integrators also ensure that the angular momenta remain within the tangent space of the quaternion coordinates. The Langevin integrators are quasi-symplectic and of weak order two. The numerical method for the gradient thermostat is of weak order one. Its construction exploits ideas of Lie-group type integrators for differential equations on manifolds. We numerically compare the discretization errors of the Langevin integrators, as well as the efficiency of the gradient integrator compared to the Langevin ones when used in the simulation of rigid TIP4P water model with smoothly truncated electrostatic interactions. We observe that the gradient integrator is computationally less efficient than the Langevin integrators. We also compare the relative accuracy of the Langevin integrators in evaluating various static quantities and give recommendations as to the choice of an appropriate integrator.Comment: 16 pages, 4 figure

Ekpyrotic collapse with multiple fields

A scale invariant spectrum of isocurvature perturbations is generated during collapse in the scaling solution in models where two or more fields have steep negative exponential potentials. The scale invariance of the spectrum is realised by a tachyonic instability in the isocurvature field. We show that this instability is due to the fact that the scaling solution is a saddle point in the phase space. The late time attractor is identified with a single field dominated ekpyrotic collapse in which a steep blue spectrum for isocurvature perturbations is found. Although quantum fluctuations do not necessarily to disrupt the classical solution, an additional preceding stage is required to establish classical homogeneity.Comment: 13 pages, 1 figur

Shubnikov-de Haas oscillations in YBa_2Cu_4O_8

We report the observation of Shubnikov-de Haas oscillations in the underdoped cuprate superconductor YBa$_2$Cu$_4$O$_8$ (Y124). For field aligned along the c-axis, the frequency of the oscillations is $660\pm 30$ T, which corresponds to $\sim 2.4$ % of the total area of the first Brillouin zone. The effective mass of the quasiparticles on this orbit is measured to be $2.7\pm0.3$ times the free electron mass. Both the frequency and mass are comparable to those recently observed for ortho-II YBa$_2$Cu$_3$O$_{6.5}$ (Y123-II). We show that although small Fermi surface pockets may be expected from band structure calculations in Y123-II, no such pockets are predicted for Y124. Our results therefore imply that these small pockets are a generic feature of the copper oxide plane in underdoped cuprates.Comment: v2: Version of paper accepted for publication in Physical Review Letters. Only minor changes to the text and reference

Exploring the role of nurses in after-hours telephone services in regional areas; A scoping review

Introduction The management of patients who need chronic and complex care is a focus of attention internationally, brought about by an increase in chronic conditions, requiring significantly more care over longer periods of time. The increase in chronic conditions has placed pressure on health services, financially and physically, bringing about changes in the way care is delivered, with hospital avoidance and home-based care encouraged. In this environment, nurses play an important role in co-ordinating care across services. This review formed one part of a funded project that explored the nurse navigator role within a proposed 24-hour telephone-call service in one regional area that has a diverse population in terms of cultural identity and geographical location in relation to service access. Aim The review reports on the extant literature on the nurse’s role in the provision of afterhours telephone services for patients with chronic and complex conditions. The specific aim was to explore the effectiveness of services for patients in geographically isolated locations. Methods The methodological approach to the review followed the Preferred Reporting System for Meta-Analyses (PRISMA) guidelines. A thematic analysis was used to identify themes with chronic care models underpinning analysis. Results Three themes were identified; nurse-led decision making; consumer profile; and program outcomes. Each theme was divided into two sub-themes. The two sub-themes for decision making were: the experience of the staff who provided the service and the tool or protocol used. The two sub-themes for consumers profile were; the geographic/demographic identity of the consumers, and consumer satisfaction. The final theme of outcomes describes how the effectiveness of the service is measured, broken into two sub-themes: the economic/workforce outcomes and the consumer outcomes. Discussion The provision of an after-hours telephone service, in whatever model used should align with a Chronic Care Model. In this way, after-hours telephone services provided by experienced nurses, supported by ongoing professional development and relevant protocols, form part of the ongoing improvement for chronic and complex care management as a health priority

Curvature perturbations from ekpyrotic collapse with multiple fields

A scale-invariant spectrum of isocurvature perturbations is generated during collapse in the ekpyrotic scaling solution in models where multiple fields have steep negative exponential potentials. The scale invariance of the spectrum is realized by a tachyonic instability in the isocurvature field. This instability drives the scaling solution to the late time attractor that is the old ekpyrotic collapse dominated by a single field. We show that the transition from the scaling solution to the single field dominated ekpyrotic collapse automatically converts the initial isocurvature perturbations about the scaling solution to comoving curvature perturbations about the late-time attractor. The final amplitude of the comoving curvature perturbation is determined by the Hubble scale at the transition.Comment: 15 pages, 3 figures, a reference added, to be published in CQG, a remark on the comoving density perturbation correcte

Undergraduate views of critical thinking

Clemson University\u27s Quality Enhanement Plan (QEP) is designed to assist undergraduates in the development of their critical thinking skills. Paul et al. (1997) found that professors indicate that they value critical thinking as important, explicit, and achieved in their students, but faculty also are vague and confusing in their open-ended descriptions of the conceptual and practical components of critical thinking instruction in the classroom. The purpose of the current study was to assess student views of the a typical professor\u27s views and student\u27s own personal views using modified versions of the survey items that Paul et al. (1997) employed. Students (n = 139) completed two eleven-item surveys; one framed a typical professor\u27s view and the other framed the student\u27s personal views of critical thinking. Students do not rate a typical professor as valuing critical thinking as important explicit and achieved in their students

Spatial partitioning of the regulatory landscape of the X-inactivation centre

In eukaryotes transcriptional regulation often involves multiple long-range elements and is influenced by the genomic environment. A prime example of this concerns the mouse X-inactivation centre (Xic), which orchestrates the initiation of X-chromosome inactivation (XCI) by controlling the expression of the non-protein-coding Xist transcript. The extent of Xic sequences required for the proper regulation of Xist remains unknown. Here we use chromosome conformation capture carbon-copy (5C) and super-resolution microscopy to analyse the spatial organization of a 4.5-megabases (Mb) region including Xist. We discover a series of discrete 200-kilobase to 1 Mb topologically associating domains (TADs), present both before and after cell differentiation and on the active and inactive X. TADs align with, but do not rely on, several domain-wide features of the epigenome, such as H3K27me3 or H3K9me2 blocks and lamina-associated domains. TADs also align with coordinately regulated gene clusters. Disruption of a TAD boundary causes ectopic chromosomal contacts and long-range transcriptional misregulation. The Xist/Tsix sense/antisense unit illustrates how TADs enable the spatial segregation of oppositely regulated chromosomal neighbourhoods, with the respective promoters of Xist and Tsix lying in adjacent TADs, each containing their known positive regulators. We identify a novel distal regulatory region of Tsix within its TAD, which produces a long intervening RNA, Linx. In addition to uncovering a new principle of cis-regulatory architecture of mammalian chromosomes, our study sets the stage for the full genetic dissection of the X-inactivation centre