510 research outputs found
Magnetic tight-binding and the iron-chromium enthalpy anomaly
We describe a self consistent magnetic tight-binding theory based in an
expansion of the Hohenberg-Kohn density functional to second order, about a non
spin polarised reference density. We show how a first order expansion about a
density having a trial input magnetic moment leads to the Stoner--Slater rigid
band model. We employ a simple set of tight-binding parameters that accurately
describes electronic structure and energetics, and show these to be
transferable between first row transition metals and their alloys. We make a
number of calculations of the electronic structure of dilute Cr impurities in
Fe which we compare with results using the local spin density approximation.
The rigid band model provides a powerful means for interpreting complex
magnetic configurations in alloys; using this approach we are able to advance a
simple and readily understood explanation for the observed anomaly in the
enthalpy of mixing.Comment: Submitted to Phys Rev
Gaussian Approximation Potentials: the accuracy of quantum mechanics, without the electrons
We introduce a class of interatomic potential models that can be
automatically generated from data consisting of the energies and forces
experienced by atoms, derived from quantum mechanical calculations. The
resulting model does not have a fixed functional form and hence is capable of
modeling complex potential energy landscapes. It is systematically improvable
with more data. We apply the method to bulk carbon, silicon and germanium and
test it by calculating properties of the crystals at high temperatures. Using
the interatomic potential to generate the long molecular dynamics trajectories
required for such calculations saves orders of magnitude in computational cost.Comment: v3-4: added new material and reference
Prediction and Observation of the bcc Structure in Pure Copper at a \u3cem\u3e\u3cstrong\u3eΣ\u3c/strong\u3e\u3c/em\u3e3 Grain Boundary
We have used molecular dynamics and simulated annealing to study an asymmetrical Σ3 tilt grain boundary with ⟨211⟩ rotation axis in Cu. The boundary plane was inclined at 84° with respect to the {}(111) plane. A simple central force N-body interatomic potential was used. The most stable configuration shows a broad band of predominantly bcc structure in the boundary region. Samples of the bicrystal with the same misorientation and inclination of the boundary plane were observed in a 1250 kV transmission electron microscope, confirming the predicted structure with atomic resolution
Electronic structure and total energy of interstitial hydrogen in iron: Tight binding models
An application of the tight binding approximation is presented for the
description of electronic structure and interatomic force in magnetic iron,
both pure and containing hydrogen impurities. We assess the simple canonical
d-band description in comparison to a non orthogonal model including s and d
bands. The transferability of our models is tested against known properties
including the segregation energies of hydrogen to vacancies and to surfaces of
iron. In many cases agreement is remarkably good, opening up the way to quantum
mechanical atomistic simulation of the effects of hydrogen on mechanical
properties
A simple environment-dependent overlap potential and Cauchy violation in solid argon
We develop an analytic and environment-dependent interatomic potential for
the overlap repulsion in solid argon, based on an approximate treatment of the
non-orthogonal Tight-Binding theory for the closed-shell systems. The present
model can well reproduce the observed elastic properties of solid argon
including Cauchy violation at high pressures, yet very simple. A useful and
novel analysis is given to show how the elastic properties are related to the
environment-dependence incorporated into a generic pairwise potential. The
present study has a close link to the broad field of computational materials
science, in which the inclusion of environment dependence in short-ranged
repulsive part of a potential model is sometimes crucial in predicting the
elastic properties correctly.Comment: 10 pages, 3 figure
The stabilizing role of itinerant ferromagnetism in inter-granular cohesion in iron
We present a simple, general energy functional for ferromagnetic materials
based upon a local spin density extension to the Stoner theory of itinerant
ferromagnetism. The functional reproduces well available ab initio results and
experimental interfacial energies for grain boundaries in iron. The model shows
that inter-granular cohesion along symmetric tilt boundaries in iron is
dependent upon strong magnetic structure at the interface, illuminates the
mechanisms underlying this structure, and provides a simple explanation for
relaxation of the atomic structure at these boundaries.Comment: In review at Phys. Rev. Lett. Submitted 23 September 1997; revised 16
March 199
Structural and chemical embrittlement of grain boundaries by impurities: a general theory and first principles calculations for copper
First principles calculations of the Sigma 5 (310)[001] symmetric tilt grain
boundary in Cu with Bi, Na, and Ag substitutional impurities provide evidence
that in the phenomenon of Bi embrittlement of Cu grain boundaries electronic
effects do not play a major role; on the contrary, the embrittlement is mostly
a structural or "size" effect. Na is predicted to be nearly as good an
embrittler as Bi, whereas Ag does not embrittle the boundary in agreement with
experiment. While we reject the prevailing view that "electronic" effects
(i.e., charge transfer) are responsible for embrittlement, we do not exclude
the role of chemistry. However numerical results show a striking equivalence
between the alkali metal Na and the semi metal Bi, small differences being
accounted for by their contrasting "size" and "softness" (defined here). In
order to separate structural and chemical effects unambiguously if not
uniquely, we model the embrittlement process by taking the system of grain
boundary and free surfaces through a sequence of precisely defined gedanken
processes; each of these representing a putative mechanism. We thereby identify
three mechanisms of embrittlement by substitutional impurities, two of which
survive in the case of embrittlement or cohesion enhancement by interstitials.
Two of the three are purely structural and the third contains both structural
and chemical elements that by their very nature cannot be further unravelled.
We are able to take the systems we study through each of these stages by
explicit computer simulations and assess the contribution of each to the nett
reduction in intergranular cohesion. The conclusion we reach is that
embrittlement by both Bi and Na is almost exclusively structural in origin;
that is, the embrittlement is a size effect.Comment: 13 pages, 5 figures; Accepted in Phys. Rev.
Development of a tight-binding potential for bcc-Zr. Application to the study of vibrational properties
We present a tight-binding potential based on the moment expansion of the
density of states, which includes up to the fifth moment. The potential is
fitted to bcc and hcp Zr and it is applied to the computation of vibrational
properties of bcc-Zr. In particular, we compute the isothermal elastic
constants in the temperature range 1200K < T < 2000K by means of standard Monte
Carlo simulation techniques. The agreement with experimental results is
satisfactory, especially in the case of the stability of the lattice with
respect to the shear associated with C'. However, the temperature decrease of
the Cauchy pressure is not reproduced. The T=0K phonon frequencies of bcc-Zr
are also computed. The potential predicts several instabilities of the bcc
structure, and a crossing of the longitudinal and transverse modes in the (001)
direction. This is in agreement with recent ab initio calculations in Sc, Ti,
Hf, and La.Comment: 14 pages, 6 tables, 4 figures, revtex; the kinetic term of the
isothermal elastic constants has been corrected (Eq. (4.1), Table VI and
Figure 4
Angular Forces Around Transition Metals in Biomolecules
Quantum-mechanical analysis based on an exact sum rule is used to extract an
semiclassical angle-dependent energy function for transition metal ions in
biomolecules. The angular dependence is simple but different from existing
classical potentials. Comparison of predicted energies with a
computer-generated database shows that the semiclassical energy function is
remarkably accurate, and that its angular dependence is optimal.Comment: Tex file plus 4 postscript figure
Natural law, non-voluntary euthanasia, and public policy
© 2019 by Emerald Publishing Limited. Natural Law philosophy asserts that there are universally binding and universally evident principles that can be determined to guide the actions of persons. Moreover, many of these principles have been enshrined in both statute and common law, thus ensuring their saliency for staff and institutions charged with palliative care. The authors examine the often emotive and politicized matter of (non-voluntary) euthanasia – acts or omissions made with the intent of causing or hastening death – with reference to Natural Law philosophy. This leads us to propose a number of important public policy remedies to ensure dignity in dying for the patient, and their associates
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