16,992 research outputs found
Free energy reconstruction from steered dynamics without post-processing
Various methods achieving importance sampling in ensembles of nonequilibrium
trajectories enable to estimate free energy differences and, by
maximum-likelihood post-processing, to reconstruct free energy landscapes.
Here, based on Bayes theorem, we propose a more direct method in which a
posterior likelihood function is used both to construct the steered dynamics
and to infer the contribution to equilibrium of all the sampled states. The
method is implemented with two steering schedules. First, using non-autonomous
steering, we calculate the migration barrier of the vacancy in Fe-alpha.
Second, using an autonomous scheduling related to metadynamics and equivalent
to temperature-accelerated molecular dynamics, we accurately reconstruct the
two-dimensional free energy landscape of the 38-atom Lennard-Jones cluster as a
function of an orientational bond-order parameter and energy, down to the
solid-solid structural transition temperature of the cluster and without
maximum-likelihood post-processing.Comment: Accepted manuscript in Journal of Computational Physics, 7 figure
Fluctuation-dissipation relations far from equilibrium
The fluctuation-dissipation (F-D) theorem is a fundamental result for systems
near thermodynamic equilibrium, and justifies studies between microscopic and
macroscopic properties. It states that the nonequilibrium relaxation dynamics
is related to the spontaneous fluctuation at equilibrium. Most processes in
Nature are out of equilibrium, for which we have limited theory. Common wisdom
believes the F-D theorem is violated in general for systems far from
equilibrium. Recently we show that dynamics of a dissipative system described
by stochastic differential equations can be mapped to that of a thermostated
Hamiltonian system, with a nonequilibrium steady state of the former
corresponding to the equilibrium state of the latter. Her we derived the
corresponding F-D theorem, and tested with several examples. We suggest further
studies exploiting the analogy between a general dissipative system appearing
in various science branches and a Hamiltonian system. Especially we discussed
the implications of this work on biological network studies.Comment: 12 pages, 4 figures, major revision over the first versio
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One-step volumetric additive manufacturing of complex polymer structures.
Two limitations of additive manufacturing methods that arise from layer-based fabrication are slow speed and geometric constraints (which include poor surface quality). Both limitations are overcome in the work reported here, introducing a new volumetric additive fabrication paradigm that produces photopolymer structures with complex nonperiodic three-dimensional geometries on a time scale of seconds. We implement this approach using holographic patterning of light fields, demonstrate the fabrication of a variety of structures, and study the properties of the light patterns and photosensitive resins required for this fabrication approach. The results indicate that low-absorbing resins containing ~0.1% photoinitiator, illuminated at modest powers (~10 to 100 mW), may be successfully used to build full structures in ~1 to 10 s
Stochastic Methods for Zero Energy Quantum Scattering
We investigate the use of stochastic methods for zero energy quantum
scattering based on a path integral approach. With the application to the
scattering of a projectile from a nuclear many body target in mind, we use the
potential scattering of a particle as a test for the accuracy and efficiency of
several methods. To be able to deal with complex potentials, we introduce a
path sampling action and a modified scattering observable. The approaches
considered are the random walk, where the points of a path are sequentially
generated, and the Langevin algorithm, which updates an entire path. Several
improvements are investigated. A cluster algorithm for dealing with scattering
problems is finally proposed, which shows the best accuracy and stability.Comment: 40 pages LaTeX, 1 Postscript file containig 20 figures; execute
main.tex file, which automatically will include other file
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