21,436 research outputs found
Coherency strain and the kinetics of phase separation in LiFePO4
A theoretical investigation of the effects of elastic coherency on the
thermodynamics, kinetics, and morphology of intercalation in single LiFePO4
nanoparticles yields new insights into this important battery material.
Anisotropic elastic stiffness and misfit strains lead to the unexpected
prediction that low-energy phase boundaries occur along {101} planes, while
conflicting reports of phase boundary orientations are resolved by a partial
loss of coherency in the {100} direction. Elastic relaxation near surfaces
leads to the formation of a striped morphology, whose characteristic length
scale is predicted by the model and yields an estimate of the interfacial
energy. The effects of coherency strain on solubility and galvanostatic
discharge are studied with a reaction-limited phase-field model, which
quantitatively captures the influence of misfit strain, particle size, and
temperature on solubility seen in experiments. Coherency strain strongly
suppresses phase separation during discharge, which enhances rate capability
and extends cycle life. The effects of elevated temperature and the feasibility
of nucleation are considered in the context of multi-particle cathodes
Theory of Nucleation in Phase-separating Nanoparticles
The basic physics of nucleation in solid \hl{single-crystal} nanoparticles is
revealed by a phase-field theory that includes surface energy, chemical
reactions and coherency strain. In contrast to binary fluids, which form
arbitrary contact angles at surfaces, complete "wetting" by one phase is
favored at binary solid surfaces. Nucleation occurs when surface wetting
becomes unstable, as the chemical energy gain (scaling with area) overcomes the
elastic energy penalty (scaling with volume). The nucleation barrier thus
decreases with the area-to-volume ratio and vanishes below a critical size, and
nanoparticles tend to transform in order of increasing size, leaving the
smallest particles homogeneous (in the phase of lowest surface energy). The
model is used to simulate phase separation in realistic nanoparticle geometries
for \ce{Li_XFePO4}, a popular cathode material for Li-ion batteries, and
collapses disparate experimental data for the nucleation barrier, with no
adjustable parameters. Beyond energy storage, the theory generally shows how to
tailor the elastic and surface properties of a solid nanostructure to achieve
desired phase behavior.Comment: 7 pages, 4 fig
Development of Novel Density Functionals for Thermochemical Kinetics
A new density functional theory (DFT) exchange-correlation functional for the
exploration of reaction mechanisms is proposed. This new functional, denoted
BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for
transition state barriers but, unlike previous attempts at such a functional,
this improved accuracy does not come at the expense of equilibrium properties.
This makes it a general-purpose functional whose domain of applicability has
been extended to transition states, rather than a specialized functional for
kinetics. The improvement in BMK rests on the inclusion of the kinetic energy
density together with a large value of the exact exchange mixing coefficient.
For this functional, the kinetic energy density appears to correct `back' the
excess exact exchange mixing for ground-state properties, possibly simulating
variable exchange.Comment: J. Chem. Phys., in press (303431JCP, scheduled for August 15, 2004
issue); supplementary data available at
http://theochem.weizmann.ac.il/web/papers/BMK.htm
Specific heat anomaly in a supercooled liquid with amorphous boundary conditions
We study the specific heat of a model supercooled liquid confined in a
spherical cavity with amorphous boundary conditions. We find the equilibrium
specific heat has a cavity-size-dependent peak as a function of temperature.
The cavity allows us to perform a finite-size scaling (FSS) analysis, which
indicates that the peak persists at a finite temperature in the thermodynamic
limit. We attempt to collapse the data onto a FSS curve according to different
theoretical scenarios, obtaining reasonable results in two cases: a
"not-so-simple" liquid with nonstandard values of the exponents {\alpha} and
{\nu}, and random first-order theory, with two different length scales.Comment: Includes Supplemental Materia
Altering the stability of the Cdc8 overlap region modulates the ability of this tropomyosin to bind cooperatively to actin and regulate myosin.
Tropomyosin (Tm) is an evolutionarily conserved ?-helical coiled-coil protein, dimers of which form end-to-end polymers capable of associating with and stabilising actin-filaments and regulate myosin function. The fission yeast, Schizosaccharomyces pombe, possesses a single essential Tm, Cdc8, which can be acetylated on its amino terminal methionine to increase its affinity for actin and enhance its ability to regulate myosin function. We have designed and generated a number of novel Cdc8 mutant proteins with amino terminal substitutions to explore how stability of the Cdc8-polymer overlap region affects the regulatory function of this Tm. By correlating the stability of each protein, its propensity to form stable polymers, its ability to associate with actin and to regulate myosin, we have shown the stability of the amino terminal of the Cdc8 ?-helix is crucial for Tm function. In addition we have identified a novel Cdc8 mutant with increased amino-terminal stability, dimers of which are capable of forming Tm-polymers significantly longer than the wild-type protein. This protein had a reduced affinity for actin with respect to wild type, and was unable to regulate actomyosin interactions. The data presented here are consistent with acetylation providing a mechanism for modulating the formation and stability of Cdc8 polymers within the fission yeast cell. The data also provide evidence for a mechanism in which Tm dimers form end-to-end polymers on the actin-filament, consistent with a cooperative model for Tm binding to actin
Triple resonant four-wavemixing boosts the yield of continuous coherent VUV generation
Continuous-wave coherent radiation in the vacuum ultraviolet (VUV)wavelength
region at 121 nm will be essential for future laser-cooling of trapped
antihydrogen [1]. Cold antihydrogen will enable both tests of the fundamental
symmetry between matter and antimatter at unprecedented experimental precision
[2] and also experiments in antimatter gravity [3]. Another fascinating
application of narrowband continuous laser radiation in the VUV is quantum
information processing using single trapped ions in Rydberg-states [4, 5]. Here
we describe highly efficient continuous four-wave mixing in the VUV by using
three different fundamental wavelengths with a sophisticated choice of
detunings to resonances of the nonlinear medium. Up to 6 microwatts of vacuum
ultraviolet radiation at 121 nm can be generated which corresponds to an
increase of three orders of magnitude in efficiency.Comment: 11 pages, 3 figure
Environmental, social, morphological, and behavioral constraints on opportunistic multiple paternity
Peer reviewedPostprin
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
The ammonia dimer (NH3)2 has been investigated using high--level ab initio
quantum chemistry methods and density functional theory (DFT). The structure
and energetics of important isomers is obtained to unprecedented accuracy
without resorting to experiment. The global minimum of eclipsed C_s symmetry is
characterized by a significantly bent hydrogen bond which deviates from
linearity by about 20 degrees. In addition, the so-called cyclic C_{2h}
structure is extremely close in energy on an overall flat potential energy
surface. It is demonstrated that none of the currently available (GGA,
meta--GGA, and hybrid) density functionals satisfactorily describe the
structure and relative energies of this nonlinear hydrogen bond. We present a
novel density functional, HCTH/407+, designed to describe this sort of hydrogen
bond quantitatively on the level of the dimer, contrary to e.g. the widely used
BLYP functional. This improved functional is employed in Car-Parrinello ab
initio molecular dynamics simulations of liquid ammonia to judge its
performance in describing the associated liquid. Both the HCTH/407+ and BLYP
functionals describe the properties of the liquid well as judged by analysis of
radial distribution functions, hydrogen bonding structure and dynamics,
translational diffusion, and orientational relaxation processes. It is
demonstrated that the solvation shell of the ammonia molecule in the liquid
phase is dominated by steric packing effects and not so much by directional
hydrogen bonding interactions. In addition, the propensity of ammonia molecules
to form bifurcated and multifurcated hydrogen bonds in the liquid phase is
found to be negligibly small.Comment: Journal of Chemical Physics, in press (305335JCP
Nepotistic patterns of violent psychopathy: evidence for adaptation?
Psychopaths routinely disregard social norms by engaging in selfish, antisocial, often violent behavior. Commonly characterized as mentally disordered, recent evidence suggests that psychopaths are executing a well-functioning, if unscrupulous strategy that historically increased reproductive success at the expense of others. Natural selection ought to have favored strategies that spared close kin from harm, however, because actions affecting the fitness of genetic relatives contribute to an individual’s inclusive fitness. Conversely, there is evidence that mental disorders can disrupt psychological mechanisms designed to protect relatives. Thus, mental disorder and adaptation accounts of psychopathy generate opposing hypotheses: psychopathy should be associated with an increase in the victimization of kin in the former account but not in the latter. Contrary to the mental disorder hypothesis, we show here in a sample of 289 violent offenders that variation in psychopathy predicts a decrease in the genetic relatedness of victims to offenders; that is, psychopathy predicts an increased likelihood of harming non-relatives. Because nepotistic inhibition in violence may be caused by dispersal or kin discrimination, we examined the effects of psychopathy on (1) the dispersal of offenders and their kin and (2) sexual assault frequency (as a window on kin discrimination). Although psychopathy was negatively associated with coresidence with kin and positively associated with the commission of sexual assault, it remained negatively associated with the genetic relatedness of victims to offenders after removing cases of offenders who had coresided with kin and cases of sexual assault from the analyses. These results stand in contrast to models positing psychopathy as a pathology, and provide support for the hypothesis that psychopathy reflects an evolutionary strategy largely favoring the exploitation of non-relatives
Psychopathy, adaptation, and disorder
In a recent study, we found a negative association between psychopathy and violence against genetic relatives. We interpreted this result as a form of nepotism and argued that it failed to support the hypothesis that psychopathy is a mental disorder, suggesting instead that it supports the hypothesis that psychopathy is an evolved life history strategy. This interpretation and subsequent arguments have been challenged in a number of ways. Here, we identify several misunderstandings regarding the harmful dysfunction definition of mental disorder as it applies to psychopathy and regarding the meaning of nepotism. Furthermore, we examine the evidence provided by our critics that psychopathy is associated with other disorders, and we offer a comment on their alternative model of psychopathy. We conclude that there remains little evidence that psychopathy is the product of dysfunctional mechanisms
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