657 research outputs found
Coarse-Graining with Equivariant Neural Networks: A Path Towards Accurate and Data-Efficient Models
Machine learning has recently entered into the mainstream of coarse-grained
(CG) molecular modeling and simulation. While a variety of methods for
incorporating deep learning into these models exist, many of them involve
training neural networks to act directly as the CG force field. This has
several benefits, the most significant of which is accuracy. Neural networks
can inherently incorporate multi-body effects during the calculation of CG
forces, and a well-trained neural network force field outperforms pairwise
basis sets generated from essentially any methodology. However, this comes at a
significant cost. First, these models are typically slower than pairwise force
fields even when accounting for specialized hardware which accelerates the
training and integration of such networks. The second, and the focus of this
paper, is the need for the considerable amount of data needed to train such
force fields. It is common to use tens of microseconds of molecular dynamics
data to train a single CG model, which approaches the point of eliminating the
CG models usefulness in the first place. As we investigate in this work, it is
apparent that this data-hunger trap from neural networks for predicting
molecular energies and forces is caused in large part by the difficulty in
learning force equivariance, i.e., the fact that force vectors should rotate
while maintaining their magnitude in response to an equivalent rotation of the
system. We demonstrate that for CG water, networks that inherently incorporate
this equivariance into their embedding can produce functional models using
datasets as small as a single frame of reference data, which networks without
inherent symmetry equivariance cannot
Reducing the Read Noise of the James Webb Space Telescope Near Infrared Spectrograph Detector Subsystem
We describe a Wiener optimal approach to using the reference output and reference pixels that are built into Teledyne's HAWAII-2RG detector arrays. In this way, we are reducing the total noise per approximately 1000 second 88 frame up-the-ramp dark integration from about 6.5 e- rms to roughly 5 e- rms. Using a principal components analysis formalism, we achieved these noise improvements without altering the hardware in any way. In addition to being lower, the noise is also cleaner with much less visible correlation. For example, the faint horizontal banding that is often seen in HAWAII-2RG images is almost completely removed. Preliminary testing suggests that the relative gains are even higher when using non flight grade components. We believe that these techniques are applicable to most HAWAII-2RG based instruments
The Gas Transfer through Polar Sea Ice Experiment: Insights into the Rates and Pathways that Determine Geochemical Fluxes
Sea ice is a defining feature of the polar marine environment. It is a critical domain for marine biota and it regulates ocean-atmosphere exchange, including the exchange of greenhouse gases such as CO2 and CH4. In this study, we determined the rates and pathways that govern gas transport through a mixed sea ice cover. N2O, SF6, 3He, 4He, and Ne were used as gas tracers of the exchange processes that take place at the ice-water and air-water interfaces in a laboratory sea ice experiment. Observation of the changes in gas concentrations during freezing revealed that He is indeed more soluble in ice than in water; Ne is less soluble in ice, and the larger gases (N2O and SF6) are mostly excluded during the freezing process. Model estimates of gas diffusion through ice were calibrated using measurements of bulk gas content in ice cores, yielding gas transfer velocity through ice (kice) of ∼5 × 10−4 m d−1. In comparison, the effective air-sea gas transfer velocities (keff) ranged up to 0.33 m d−1 providing further evidence that very little mixed-layer ventilation takes place via gas diffusion through columnar sea ice. However, this ventilation is distinct from air-ice gas fluxes driven by sea ice biogeochemistry. The magnitude of keff showed a clear increasing trend with wind speed and current velocity beneath the ice, as well as the combination of the two. This result indicates that gas transfer cannot be uniquely predicted by wind speed alone in the presence of sea ice
Predator-specific inducible morphological defenses of a water flea against two freshwater predators
Unusual magnetic-field dependence of partially frustrated triangular ordering in manganese tricyanomethanide
Manganese tricyanomethanide, Mn[C(CN)3]2, consists of two interpenetrating
three-dimensional rutile-like networks. In each network, the tridentate C(CN)3-
anion gives rise to superexchange interactions between the Mn2+ ions (S=5/2)
that can be mapped onto the "row model" for partially frustrated triangular
magnets. We present heat capacity measurements that reveal a phase transition
at T_N = 1.18K, indicative of magnetic ordering. The zero-field magnetically
ordered structure was solved from neutron powder diffraction data taken between
0.04 and 1.2 K. It consists of an incommensurate spiral with a temperature
independent propagation vector Q=(2Q 0 0)=(+/-0.622 0 0), where different signs
relate to the two different networks. This corresponds to (+/-0.311 +/-0.311 0)
in a quasi-hexagonal representation. The ordered moment mu=3.3mu_B is about 2/3
of the full Mn2+ moment. From the values of T_N and Q, the exchange parameters
J/k = 0.15 K and J'/J = 0.749 are estimated. The magnetic-field dependence of
the intensity of the Bragg reflection, measured for external fields
H||Q, indicates the presence of three different magnetic phases. We associate
them with the incommensurate spiral (H < 13.5 kOe), an intermediate phase (13.5
kOe 16 kOe)
proposed for related compounds. For increasing fields, Q continuously
approaches the value 1/3, corresponding to the commensurate magnetic structure
of the fully frustrated triangular lattice. This value is reached at H_c = 19
kOe. At this point, the field-dependence reverses and Q adopts a value of 0.327
at 26 kOe, the highest field applied in the experiment. Except for H_c, the
magnetic ordering is incommensurate in all three magnetic phases of
Mn[C(CN)3]2.Comment: accepted for publication in J. Phys.: Condens. Matte
Measurements and analysis of the upper critical field on an underdoped and overdoped compounds
The upper critical field is one of the many non conventional
properties of high- cuprates. It is possible that the
anomalies are due to the presence of inhomogeneities in the local charge
carrier density of the planes. In order to study this point, we
have prepared good quality samples of polycrystalline
using the wet-chemical method, which has demonstrated to produce samples with a
better cation distribution. In particular, we have studied the temperature
dependence of the second critical field, , through the magnetization
measurements on two samples with opposite average carrier concentration
() and nearly the same critical temperature, namely
(underdoped) and (overdoped). The results close to do not
follow the usual Ginzburg-Landau theory and are interpreted by a theory which
takes into account the influence of the inhomogeneities.Comment: Published versio
Protein Pattern Formation
Protein pattern formation is essential for the spatial organization of many
intracellular processes like cell division, flagellum positioning, and
chemotaxis. A prominent example of intracellular patterns are the oscillatory
pole-to-pole oscillations of Min proteins in \textit{E. coli} whose biological
function is to ensure precise cell division. Cell polarization, a prerequisite
for processes such as stem cell differentiation and cell polarity in yeast, is
also mediated by a diffusion-reaction process. More generally, these functional
modules of cells serve as model systems for self-organization, one of the core
principles of life. Under which conditions spatio-temporal patterns emerge, and
how these patterns are regulated by biochemical and geometrical factors are
major aspects of current research. Here we review recent theoretical and
experimental advances in the field of intracellular pattern formation, focusing
on general design principles and fundamental physical mechanisms.Comment: 17 pages, 14 figures, review articl
Reducing the Read Noise of HAWAII-2RG Detector Systems with Improved Reference Sampling and Subtraction (IRS2)
IRS2 is a Wiener-optimal approach to using all of the reference information that Teledyne's HAWAII-2RG detector arrays provide. Using a new readout pattern, IRS2 regularly interleaves reference pixels with the normal pixels during readout. This differs from conventional clocking, in which the reference pixels are read out infrequently, and only in a few rows and columns around the outside edges of the detector array. During calibration, the data are processed in Fourier space, which is <;:lose to the noise's eigenspace. Using IRS2, we have reduced the read noise of the James Webb Space Telescope Near Infrared Spectrograph by 15% compared to conventional readout. We are attempting to achieve further gains by calibrating out recently recognized non-stationary noise that appears at the frame rate
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