11,451 research outputs found

### 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 $C_2 D^{-2}$ vdW power law while exfoliation
obeys a $C_3 \log(D/D_0) D^{-3}$ law for graphitics and a $C_{5/2} D^{-5/2}$
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 $C_6 D^{-6}$' type vdW methods might be valid such as
layered bulk to layered bulk and layered bulk to atom

### A theoretical and semiemprical correction to the long-range dispersion power law of stretched graphite

In recent years intercalated and pillared graphitic systems have come under
increasing scrutiny because of their potential for modern energy technologies.
While traditional \emph{ab initio} methods such as the LDA give accurate
geometries for graphite they are poorer at predicting physicial properties such
as cohesive energies and elastic constants perpendicular to the layers because
of the strong dependence on long-range dispersion forces. `Stretching' the
layers via pillars or intercalation further highlights these weaknesses. We use
the ideas developed by [J. F. Dobson et al, Phys. Rev. Lett. {\bf 96}, 073201
(2006)] as a starting point to show that the asymptotic $C_3 D^{-3}$ dependence
of the cohesive energy on layer spacing $D$ in bigraphene is universal to all
graphitic systems with evenly spaced layers. At spacings appropriate to
intercalates, this differs from and begins to dominate the $C_4 D^{-4}$ power
law for dispersion that has been widely used previously. The corrected power
law (and a calculated $C_3$ coefficient) is then unsuccesfully employed in the
semiempirical approach of [M. Hasegawa and K. Nishidate, Phys. Rev. B {\bf 70},
205431 (2004)] (HN). A modified, physicially motivated semiempirical method
including some $C_4 D^{-4}$ effects allows the HN method to be used
successfully and gives an absolute increase of about $2-3%$ to the predicted
cohesive energy, while still maintaining the correct $C_3 D^{-3}$ asymptotics

### Nonuniversality of the dispersion interaction: analytic benchmarks for van der Waals energy functionals

We highlight the non-universality of the asymptotic behavior of dispersion
forces, such that a sum of inverse sixth power contributions is often
inadequate. We analytically evaluate the cross-correlation energy Ec between
two pi-conjugated layers separated by a large distance D within the
electromagnetically non-retarded Random Phase Approximation, via a
tight-binding model. For two perfect semimetallic graphene sheets at T=0K we
find Ec = C D^{-3}, in contrast to the "insulating" D^{-4} dependence predicted
by currently accepted approximations. We also treat the case where one graphene
layer is replaced by a thin metal, a model relevant to the exfoliation of
graphite. Our general considerations also apply to nanotubes, nanowires and
layered metals.Comment: 4 pages, 0 fig

### Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures

Recent investigations have highlighted the failure of a sum of $R^{-6}$ terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex

### Self-Templated Nucleation in Peptide and Protein aggregation

Peptides and proteins exhibit a common tendency to assemble into highly
ordered fibrillar aggregates, whose formation proceeds in a
nucleation-dependent manner that is often preceded by the formation of
disordered oligomeric assemblies. This process has received much attention
because disordered oligomeric aggregates have been associated with
neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Here
we describe a self-templated nucleation mechanism that determines the
transition between the initial condensation of polypeptide chains into
disordered assemblies and their reordering into fibrillar structures. The
results that we present show that at the molecular level this transition is due
to the ability of polypeptide chains to reorder within oligomers into fibrillar
assemblies whose surfaces act as templates that stabilise the disordered
assemblies.Comment: 4 pages, 3 figure

### High-Level Correlated Approach to the Jellium Surface Energy, Without Uniform-Electron-Gas Input

We resolve the long-standing controversy over the surface energy of simple
metals: Density functional methods that require uniform-electron-gas input
agree with each other at many levels of sophistication, but not with high-level
correlated calculations like Fermi Hypernetted Chain and Diffusion Monte Carlo
(DMC) that predict the uniform-gas correlation energy. Here we apply a very
high-level correlated approach, the inhomogeneous Singwi-Tosi-Land-Sj\"olander
(ISTLS) method, and find that the density functionals are indeed reliable
(because the surface energy is "bulk-like"). ISTLS values are close to
recently-revised DMC values. Our work also vindicates the previously-disputed
use of uniform-gas-based nonlocal kernels in time-dependent density functional
theory.Comment: 4 pages, 1 figur

### Beyond the Random Phase Approximation for the Electron Correlation Energy: The Importance of Single Excitations

The random phase approximation (RPA) for the electron correlation energy,
combined with the exact-exchange energy, represents the state-of-the-art
exchange-correlation functional within density-functional theory (DFT).
However, the standard RPA practice -- evaluating both the exact-exchange and
the RPA correlation energy using local or semilocal Kohn-Sham (KS) orbitals --
leads to a systematic underbinding of molecules and solids. Here we demonstrate
that this behavior is largely corrected by adding a "single excitation" (SE)
contribution, so far not included in the standard RPA scheme. A similar
improvement can also be achieved by replacing the non-self-consistent
exact-exchange total energy by the corresponding self-consistent Hartree-Fock
total energy, while retaining the RPA correlation energy evaluated using
Kohn-Sham orbitals. Both schemes achieve chemical accuracy for a standard
benchmark set of non-covalent intermolecular interactions.Comment: 5 pages, 4 figures, and an additional supplementary materia

### Many-body approach to infinite non-periodic systems: application to the surface of semi-infinite jellium

A method to implement the many-body Green function formalism in the GW
approximation for infinite non periodic systems is presented. It is suitable to
treat systems of known ``asymptotic'' properties which enter as boundary
conditions, while the effects of the lower symmetry are restricted to regions
of finite volume. For example, it can be applied to surfaces or localized
impurities. We illustrate the method with a study of the surface of
semi-infinite jellium. We report the dielectric function, the effective
potential and the electronic self-energy discussing the effects produced by the
screening and by the charge density profile near the surface.Comment: 11 pages, 4 figure

### Correlation potentials for molecular bond dissociation within the self-consistent random phase approximation

Self-consistent correlation potentials for H$_2$ and LiH for various
inter-atomic separations are obtained within the random phase approximation
(RPA) of density functional theory. The RPA correlation potential shows a peak
at the bond midpoint, which is an exact feature of the true correlation
potential, but lacks another exact feature: the step important to preserve
integer charge on the atomic fragments in the dissociation limit. An analysis
of the RPA energy functional in terms of fractional charge is given which
confirms these observations. We find that the RPA misses the derivative
discontinuity at odd integer particle numbers but explicitly eliminates the
fractional spin error in the exact-exchange functional. The latter finding
explains the accurate total energy in the dissociation limit.Comment: 9 pages, 10 figure

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