Time step rescaling recovers continuous-time dynamical properties for discrete-time Langevin integration of nonequilibrium systems

When simulating molecular systems using deterministic equations of motion (e.g., Newtonian dynamics), such equations are generally numerically integrated according to a well-developed set of algorithms that share commonly agreed-upon desirable properties. However, for stochastic equations of motion (e.g., Langevin dynamics), there is still broad disagreement over which integration algorithms are most appropriate. While multiple desiderata have been proposed throughout the literature, consensus on which criteria are important is absent, and no published integration scheme satisfies all desiderata simultaneously. Additional nontrivial complications stem from simulating systems driven out of equilibrium using existing stochastic integration schemes in conjunction with recently-developed nonequilibrium fluctuation theorems. Here, we examine a family of discrete time integration schemes for Langevin dynamics, assessing how each member satisfies a variety of desiderata that have been enumerated in prior efforts to construct suitable Langevin integrators. We show that the incorporation of a novel time step rescaling in the deterministic updates of position and velocity can correct a number of dynamical defects in these integrators. Finally, we identify a particular splitting that has essentially universally appropriate properties for the simulation of Langevin dynamics for molecular systems in equilibrium, nonequilibrium, and path sampling contexts.Comment: 15 pages, 2 figures, and 2 table

Connections between efficient control and spontaneous transitions in an Ising model

A system can be driven between metastable configurations by a time-dependent driving protocol, which uses external control parameters to change the potential energy of the system. Here we investigate the correspondence between driving protocols that are designed to minimize work and the spontaneous transition paths of the system in the absence of driving. We study the spin-inversion reaction in a 2D Ising model, quantifying the timing of each spin flip and heat flow to the system during both a minimum-work protocol and a spontaneous transition. The general order of spin flips during the transition mechanism is preserved between the processes, despite the coarseness of control parameters that are unable to reproduce more detailed features of the spontaneous mechanism. Additionally, external control parameters provide energy to each system component to compensate changes in internal energy, showing how control parameters are tuned during a minimum-work protocol to counteract underlying energetic features. This study supports a correspondence between minimum-work protocols and spontaneous transition mechanisms.Comment: 10 pages, 6 figure

Performance Effects of Adding a Parallel Capacitor to a Pulse Inductive Plasma Accelerator Powertrain

Pulsed inductive plasma accelerators are electrodeless space propulsion devices where a capacitor is charged to an initial voltage and then discharged through a coil as a high-current pulse that inductively couples energy into the propellant. The field produced by this pulse ionizes the propellant, producing a plasma near the face of the coil. Once a plasma is formed if can be accelerated and expelled at a high exhaust velocity by the Lorentz force arising from the interaction of an induced plasma current and the magnetic field. While there are many coil geometries that can be employed to inductively accelerate a plasma, in this paper the discussion is limit to planar geometries where the coil take the shape of a flat spiral. A recent review of the developmental history of planar-geometry pulsed inductive thrusters can be found in Ref. [1]. Two concepts that have employed this geometry are the Pulsed Inductive Thruster (PIT) and the Faraday Accelerator with Radio-frequency Assisted Discharge (FARAD)

George C. Marshall Space Flight Center Research and Technology Report 2014

Many of NASA's missions would not be possible if it were not for the investments made in research advancements and technology development efforts. The technologies developed at Marshall Space Flight Center contribute to NASA's strategic array of missions through technology development and accomplishments. The scientists, researchers, and technologists of Marshall Space Flight Center who are working these enabling technology efforts are facilitating NASA's ability to fulfill the ambitious goals of innovation, exploration, and discovery

Marshall Space Flight Center Research and Technology Report 2015

The investments in technology development we made in 2015 not only support the Agency's current missions, but they will also enable new missions. Some of these projects will allow us to develop an in-space architecture for human space exploration; Marshall employees are developing and testing cutting-edge propulsion solutions that will propel humans in-space and land them on Mars. Others are working on technologies that could support a deep space habitat, which will be critical to enable humans to live and work in deep space and on other worlds. Still others are maturing technologies that will help new scientific instruments study the outer edge of the universe-instruments that will provide valuable information as we seek to explore the outer planets and search for life

Thermodynamic metrics and optimal paths

A fundamental problem in modern thermodynamics is how a molecular-scale machine performs useful work, while operating away from thermal equilibrium without excessive dissipation. To this end, we derive a friction tensor that induces a Riemannian manifold on the space of thermodynamic states. Within the linear-response regime, this metric structure controls the dissipation of finite-time transformations, and bestows optimal protocols with many useful properties. We discuss the connection to the existing thermodynamic length formalism, and demonstrate the utility of this metric by solving for optimal control parameter protocols in a simple nonequilibrium model.Comment: 5 page

Return to driving after traumatic brain injury : a British perspective

Primary Objective: to identify current legal situation, and professional practice in assisting persons with traumatic brain injury (TBI) to return to safe driving after injury. Methods and Procedures A brief review of relevant literature, a description of the current statutory and quasi-statutory authorities regulating return to driving after TBI in the UK, and a description of the nature and resolution of clinical and practical dilemmas facing professionals helping return to safe driving after TBI. Each of the 15 UK mobility centres was contacted and literature requested; in addition a representative of each centre responded to a structured telephone survey. Main Outcome and Results: The current situation in Great Britain is described, with a brief analysis of the strengths and weaknesses both of the current statutory situation, and also the practical situation (driving centres), with suggestions for improvements in practice. Conclusion Although brain injury may cause serious limitations in driving ability, previous drivers are not routinely assessed or advised regarding return to driving after TBI

Controlling DNA capture and propagation through artificial nanopores

Electrophorescing blopolymers across nanopores modulates the ionic current through the pore, revealing the polymer's diameter, length, and conformation. The rapidity of polymer translocation (similar to 30 000 bp/ms) in this geometry greatly limits the information that can be obtained for each base. Here we show that the translocation speed of lambda-DNA through artificial nanopores can be reduced using. optical tweezers. DNAs coupled to optically trapped beads were presented to nanopores. DNAs initially placed up to several micrometers from the pore could be captured. Subsequently, the optical tweezers reduced translocation speeds to 150 bp/ms, about 200-fold slower than free DNA. Moreover, the optical tweezers allowed us to "floss" single polymers back and forth through the pore. The combination of controlled sample presentation, greatly slowed translocation speeds, and repeated electrophoresis of single DNAs removes several barriers to using artificial nanopores for sequencing, haplotyping, and characterization of protein-DNA interactions