58,860 research outputs found
On the Accuracy of van der Waals Inclusive Density-Functional Theory Exchange-Correlation Functionals for Ice at Ambient and High Pressures
Density-functional theory (DFT) has been widely used to study water and ice
for at least 20 years. However, the reliability of different DFT
exchange-correlation (xc) functionals for water remains a matter of
considerable debate. This is particularly true in light of the recent
development of DFT based methods that account for van der Waals (vdW)
dispersion forces. Here, we report a detailed study with several xc functionals
(semi-local, hybrid, and vdW inclusive approaches) on ice Ih and six proton
ordered phases of ice. Consistent with our previous study [Phys. Rev. Lett.
107, 185701 (2011)] which showed that vdW forces become increasingly important
at high pressures, we find here that all vdW inclusive methods considered
improve the relative energies and transition pressures of the high-pressure ice
phases compared to those obtained with semi-local or hybrid xc functionals.
However, we also find that significant discrepancies between experiment and the
vdW inclusive approaches remain in the cohesive properties of the various
phases, causing certain phases to be absent from the phase diagram. Therefore,
room for improvement in the description of water at ambient and high pressures
remains and we suggest that because of the stern test the high pressure ice
phases pose they should be used in future benchmark studies of simulation
methods for water.Comment: 13 pages, 5 figures, 4 table
Challenges in first-principles NPT molecular dynamics of soft porous crystals: A case study on MIL-53(Ga)
Soft porous crystals present a challenge to molecular dynamics simulations
with flexible size and shape of the simulation cell (i.e., in the NPT
ensemble), since their framework responds very sensitively to small external
stimuli. Hence, all interactions have to be described very accurately in order
to obtain correct equilibrium structures. Here, we report a methodological
study on the nanoporous metal-organic framework MIL-53(Ga), which undergoes a
large-amplitude transition between a narrow- and a large-pore phase upon a
change in temperature. Since this system has not been investigated by density
functional theory (DFT)-based NPT simulations so far, we carefully check the
convergence of the stress tensor with respect to computational parameters.
Furthermore, we demonstrate the importance of dispersion interactions and test
two different ways of incorporating them into the DFT framework. As a result,
we propose two computational schemes which describe accurately the narrow- and
the large-pore phase of the material, respectively. These schemes can be used
in future work on the delicate interplay between adsorption in the nanopores
and structural flexibility of the host material
Error estimates for solid-state density-functional theory predictions: an overview by means of the ground-state elemental crystals
Predictions of observable properties by density-functional theory
calculations (DFT) are used increasingly often in experimental condensed-matter
physics and materials engineering as data. These predictions are used to
analyze recent measurements, or to plan future experiments. Increasingly more
experimental scientists in these fields therefore face the natural question:
what is the expected error for such an ab initio prediction? Information and
experience about this question is scattered over two decades of literature. The
present review aims to summarize and quantify this implicit knowledge. This
leads to a practical protocol that allows any scientist - experimental or
theoretical - to determine justifiable error estimates for many basic property
predictions, without having to perform additional DFT calculations. A central
role is played by a large and diverse test set of crystalline solids,
containing all ground-state elemental crystals (except most lanthanides). For
several properties of each crystal, the difference between DFT results and
experimental values is assessed. We discuss trends in these deviations and
review explanations suggested in the literature. A prerequisite for such an
error analysis is that different implementations of the same first-principles
formalism provide the same predictions. Therefore, the reproducibility of
predictions across several mainstream methods and codes is discussed too. A
quality factor Delta expresses the spread in predictions from two distinct DFT
implementations by a single number. To compare the PAW method to the highly
accurate APW+lo approach, a code assessment of VASP and GPAW with respect to
WIEN2k yields Delta values of 1.9 and 3.3 meV/atom, respectively. These
differences are an order of magnitude smaller than the typical difference with
experiment, and therefore predictions by APW+lo and PAW are for practical
purposes identical.Comment: 27 pages, 20 figures, supplementary material available (v5 contains
updated supplementary material
Developing and assessing a new web-based tapping test for measuring distal movement in Parkinson's disease: a Distal Finger Tapping test
Disability in Parkinson's disease (PD) is measured by standardised scales including the MDS-UPDRS, which are subject to high inter and intra-rater variability and fail to capture subtle motor impairment. The BRadykinesia Akinesia INcoordination (BRAIN) test is a validated keyboard tapping test, evaluating proximal upper-limb motor impairment. Here, a new Distal Finger Tapping (DFT) test was developed to assess distal upper-limb function. Kinetic parameters of the test include kinesia score (KS20, key taps over 20 s), akinesia time (AT20, mean dwell-time on each key) and incoordination score (IS20, variance of travelling time between key taps). To develop and evaluate a new keyboard-tapping test for objective and remote distal motor function in PD patients. The DFT and BRAIN tests were assessed in 55 PD patients and 65 controls. Test scores were compared between groups and correlated with the MDS-UPDRS-III finger tapping sub-scores. Nine additional PD patients were recruited for monitoring motor fluctuations. All three parameters discriminated effectively between PD patients and controls, with KS20 performing best, yielding 79% sensitivity for 85% specificity; area under the receiver operating characteristic curve (AUC)â=â0.90. A combination of DFT and BRAIN tests improved discrimination (AUCâ=â0.95). Among three parameters, KS20 showed a moderate correlation with the MDS-UPDRS finger-tapping sub-score (Pearson's râ=â- 0.40, pâ=â0.002). Further, the DFT test detected subtle changes in motor fluctuation states which were not reflected clearly by the MDS-UPDRS-III finger tapping sub-scores. The DFT test is an online tool for assessing distal movements in PD, with future scope for longitudinal monitoring of motor complications
A design for testability study on a high performance automatic gain control circuit.
A comprehensive testability study on a commercial automatic gain control circuit is presented which aims to identify design for testability (DfT) modifications to both reduce production test cost and improve test quality. A fault simulation strategy based on layout extracted faults has been used to support the study. The paper proposes a number of DfT modifications at the layout, schematic and system levels together with testability. Guidelines that may well have generic applicability. Proposals for using the modifications to achieve partial self test are made and estimates of achieved fault coverage and quality levels presente
Use of differential framing to measure implicit social cognitions associated with aggression
The purpose of this dissertation was to respond to the call offered by James (1998; James & Mazerolle, 2002; James et al., 2001) for the development and validation of new indirect measurement systems applicable to implicit social cognitions. Such a measurement system is described herein. This measurement system is based on the notion of differential framingâthat is, the idea that individuals with different personalities tend to frame the same situations and stimuli in qualitatively different manners. A test based on differential framing was developed to assess framing proclivities associated with dispositional aggression. This test is called the Differential Framing Test or DFT. Data were collected and analyzed from four different samples. The DFT demonstrated strong predictive validity yielding cross-validities in the .305 and .403. Furthermore, and in direct contrast to many indirect measurement systems (e. g., Thematic Apperception Test), the current measurement system demonstrated appropriate levels of internal consistency and test-retest reliability, given its early stage of development. Overall, it appears that differential framing represents a viable approach to measuring personality-related implicit cognitions. Furthermore, this approach yields results that are not redundant with other measures of implicit social cognitions (e. g., Conditional Reasoning Tests). Results are discussed in light of directions for future research
Investigating the Feasibility of Integrating Pavement Friction and Texture Depth Data in Modeling for INDOT PMS
Under INDOTâs current friction testing program, the friction is measured annually on interstates but only once every three years on non-interstate roadways. The stateâs Pavement Management System, however, would require current data if friction were to be included in the PMS. During routine pavement condition monitoring for the PMS, texture data is collected annually. This study explored the feasibility of using this pavement texture data to estimate the friction during those years when friction is not measured directly. After multi0ple approaches and a wide variety of ways of examining the currently available data and texture measuring technologies, it was determined that it is not currently feasible to use the texture data as a surrogate for friction testing. This is likely because the lasers used at this time are not capable of capturing the small-scale pavement microtexture. This situation may change, however, with advances in laser or photo interpretation technologies and improved access to materials data throughout the INDOT pavement network
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