14,440 research outputs found
Efficient, long-range correlation from occupied wavefunctions only
We use continuum mechanics [Tao \emph{et al}, PRL{\bf 103},086401] to
approximate the dynamic density response of interacting many-electron systems.
Thence we develop a numerically efficient exchange-correlation energy
functional based on the Random Phase Approximation (dRPA). The resulting
binding energy curve for thin parallel metal slabs at separation
better agrees with full dRPA calculations than does the Local Density
Approximation. We also reproduce the correct non-retarded van der Waals (vdW)
power law E(D)\aeq -C_{5/2}D^{-5/2} as , unlike most vdW
functionals.Comment: 4 pages, 1 figur
The flexible nature of exchange, correlation and Hartree physics: resolving "delocalization" errors in a 'correlation free' density functional
By exploiting freedoms in the definitions of 'correlation', 'exchange' and
'Hartree' physics in ensemble systems we better generalise the notion of 'exact
exchange' (EXX) to systems with fractional occupations functions of the
frontier orbitals, arising in the dissociation limit of some molecules. We
introduce the Linear EXX ("LEXX") theory whose pair distribution and energy are
explicitly \emph{piecewise linear} in the occupations . {\hi}We
provide explicit expressions for these functions for frontier and
shells. Used in an optimised effective potential (OEP) approach it yields
energies bounded by the piecewise linear 'ensemble EXX' (EEXX) energy and
standard fractional optimised EXX energy: .
Analysis of the LEXX explains the success of standard OEP methods for diatoms
at large spacing, and why they can fail when both spins are allowed to be
non-integer so that "ghost" Hartree interactions appear between \emph{opposite}
spin electrons in the usual formula. The energy contains a
cancellation term for the spin ghost case. It is evaluated for H, Li and Na
fractional ions with clear derivative discontinuities for all cases. The
-shell form reproduces accurate correlation-free energies of B-F and Al-Cl.
We further test LEXX plus correlation energy calculations on fractional ions of
C and F and again shows both derivative discontinuities and good agreement with
exact results
Dispersion corrections in graphenic systems: a simple and effective model of binding
We combine high-level theoretical and \emph{ab initio} understanding of
graphite to develop a simple, parametrised force-field model of interlayer
binding in graphite, including the difficult non-pairwise-additive
coupled-fluctuation dispersion interactions. The model is given as a simple
additive correction to standard density functional theory (DFT) calculations,
of form where is the interlayer
distance. The functions are parametrised by matching contact properties, and
long-range dispersion to known values, and the model is found to accurately
match high-level \emph{ab initio} results for graphite across a wide range of
values. We employ the correction on the difficult bigraphene binding and
graphite exfoliation problems, as well as lithium intercalated graphite
LiC. We predict the binding energy of bigraphene to be 0.27 J/m^2, and the
exfoliation energy of graphite to be 0.31 J/m^2, respectively slightly less and
slightly more than the bulk layer binding energy 0.295 J/m^2/layer. Material
properties of LiC are found to be essentially unchanged compared to the
local density approximation. This is appropriate in view of the relative
unimportance of dispersion interactions for LiC layer binding
van der Waals dispersion power laws for cleavage, exfoliation and stretching in multi-scale, layered systems
Layered and nanotubular systems that are metallic or graphitic are known to
exhibit unusual dispersive van der Waals (vdW) power laws under some
circumstances. In this letter we investigate the vdW power laws of bulk and
finite layered systems and their interactions with other layered systems and
atoms in the electromagnetically non-retarded case. The investigation reveals
substantial difference between `cleavage' and `exfoliation' of graphite and
metals where cleavage obeys a vdW power law while exfoliation
obeys a law for graphitics and a
law for layered metals. This leads to questions of relevance in the
interpretation of experimental results for these systems which have previously
assumed more trival differences. Furthermore we gather further insight into the
effect of scale on the vdW power laws of systems that simultaneously exhibit
macroscopic and nanoscopic dimensions. We show that, for metallic and graphitic
layered systems, the known "unusual" power laws can be reduced to standard or
near standard power laws when the effective scale of one or more dimension is
changed. This allows better identification of the systems for which the
commonly employed `sum of ' type vdW methods might be valid such as
layered bulk to layered bulk and layered bulk to atom
How many-body effects modify the van der Waals interaction between graphene sheets
Undoped graphene (Gr) sheets at low temperatures are known, via Random Phase
Approximation (RPA) calculations, to exhibit unusual van der Waals (vdW)
forces. Here we show that graphene is the first known system where effects
beyond the RPA make qualitative changes to the vdW force. For large
separations, nm where only the vdW forces remain, we find
the Gr-Gr vdW interaction is substantially reduced from the RPA prediction. Its
dependence is very sensitive to the form of the long-wavelength many-body
enhancement of the velocity of the massless Dirac fermions, and may provide
independent confirmation of the latter via direct force measurements.Comment: 3 Figures: PACS 73.22.Pr, 71.10.Pm, 61.48.Gh, 34.20.C
Does Terrorism Work?
This paper examines whether terrorism is an effective tool to achieve political goals. By exploiting variation in terror attacks over time and across locations in Israel from 1984 to 2006, we show that local terror attacks cause Israelis to be: (i) more willing to grant territorial concessions to the Palestinians; (ii) more willing to accept a Palestinian state; (iii) less likely to identify oneself as being right-wing; and (iv) more likely to have a favorable opinion of Arabs. These effects are especially pronounced for individuals from particular demographic groups which are typically right-wing in their political views. In addition, we show that terror induces Israelis to vote increasingly for right-wing parties. This pattern of results demonstrates that right-wing parties are becoming more accommodating to Palestinian demands for territorial concessions. Hence, this paper shows that terrorism appears to be an effective strategy in terms of shifting the entire Israeli political landscape to the left. These findings may shed light on the causes underlying the spread of global terrorism in the last few decades.
Thermally excited fluctuations as a pure electron plasma temperature diagnostic
Thermally excited charge fluctuations in pure electron plasma columns provide a diagnostic for the plasma temperature over a range of 0.05 0.2, so that Landau damping is dominant and well modeled by theory. The third method compares the total (frequency-integrated) number delta N of fluctuating image charges on the wall antenna to a simple thermodynamic calculation. This method works when lambda(D)/R-p > 0.2
Thermally excited Trivelpiece–Gould modes as a pure electron plasma temperature diagnostic
Thermally excited plasma modes are observed in trapped, near-thermal-equilibrium pure electron plasmas over a temperature range of 0.05<kT<5 eV. The modes are excited and damped by thermal fluctuations in both the plasma and the receiver electronics. The thermal emission spectra together with a plasma-antenna coupling coefficient calibration uniquely determine the plasma (and load) temperature. This calibration is obtained from the mode spectra themselves when the receiver-generated noise absorption is measurable; or from separate wave reflection/absorption measurements; or from kinetic theory. This nondestructive temperature diagnostic agrees well with standard diagnostics, and may be useful for expensive species such as antimatter
Does post-mining rehabilitation on the Weipa bauxite plateau restore bird habitat values?
Rehabilitation is seen as a key strategy for minimising biodiversity losses. Although most rehabilitation strategies aim to provide habitat for fauna, they usually focus entirely on establishing vegetation. Successful vegetation establishment, however, does not necessarily provide habitat to the same species that are threatened by habitat loss. Improved understanding of faunal response to rehabilitation is required if rehabilitation techniques are to be refined and deliver the hoped for biodiversity outcomes. This study aimed to assess to what extent post-mining rehabilitation on the Weipa bauxite plateau has restored the bird habitat values of the pre-mining native forest. Bird assemblages, vegetation, and landscape functionality were compared between: (1) Eucalyptus tetrodonta open forest reference sites representative of the pre-mining native forest; (2) two reference land units of Eucalyptus tetrodonta tall woodland that have previously been nominated as ecologically appropriate analogues for the post-mining landscapes; and (3) a chronosequence of post-mining rehabilitation sites up to 23 years old. Bird species richness and mean bird abundance increased with rehabilitation age. Bird species composition also changed and became more similar to native forest bird assemblages with increasing age. Significant differences remained, however, in mean bird abundance and composition of the bird assemblages between the oldest age class of mine rehabilitation and reference native forest land units. The mean bird species shortfall index in the oldest age class of mine rehabilitation was 63%, compared to a mean species shortfall index of 27% for pre-mining native forest sites. There were also significant differences in vegetation composition and structure between reference native forest land units and post-mining rehabilitation sites. Most importantly, the framework plant species that dominate the native vegetation community occurred at much lower densities in mine rehabilitation than in reference land units. Site detection rates of birds were strongly related to vegetation composition and structure.
It is concluded that mining and post-mining rehabilitation on the Weipa bauxite plateau has so far resulted in habitat conversion rather than habitat restoration. It therefore contributes to the causes of biodiversity decline. Post-mining rehabilitation created new habitat for 18 bird species not sourced from the pre-mining native forest. It also provided partial habitat for many of the generalist native forest bird species recorded, although their presence in the landscape remained dependent on access to native forest. However, rehabilitation did not provide the habitat resources that are required by habitat specialists and foraging specialists. This study found that the native forest bird species most sensitive to habitat loss, and most in need of habitat restoration, may be the last to return to rehabilitation if they return at all. The findings of this study have implications for rehabilitation practices, biodiversity conservation on the Weipa bauxite plateau, as well as broader implications for policies that rely on the assumption that rehabilitation can offset biodiversity losses.ANU Graduate School Scholarship. Rio Tinto Aluminium Weip
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