4,445 research outputs found
Nonlinear instability of density-independent orbital-free kinetic energy functionals
We study in this article the mathematical properties of a class of
orbital-free kinetic energy functionals. We prove that these models are
linearly stable but nonlinearly unstable, in the sense that the corresponding
kinetic energy functionals are not bounded from below. As a matter of
illustration, we provide an example of an electronic density of simple shape
the kinetic energy of which is negative.Comment: 14 pages, 1 figur
The effects of temperature gradient and growth rate on the morphology and fatigue properties of MAR-M246(Hf)
MAR-M246(Hf) is a nickel based superalloy used in the turbopump blades of the Space Shuttle main engines. The effects are considered of temperature gradient (G) and growth rate (R) on the microstructure and fatigue properties of this superalloy. The primary dendrite arm spacings were found to be inversely proportional to both temperature gradient and growth rate. Carbide and gamma - gamma prime morphology trends were related to G/R ratios. Weibull analysis of fatigue results shows the characteristic life to be larger by a factor of 10 for the low gradient/fast rate pairing of G and R, while the reliability (beta) was lower
Electron correlation in solids via density embedding theory
Density matrix embedding theory (Phys. Rev. Lett. 109, 186404 (2012)) and
density embedding theory ((Phys. Rev. B 89, 035140 (2014)) have recently been
introduced for model lattice Hamiltonians and molecular systems. In the present
work, the formalism is extended to the ab initio description of infinite
systems. An appropriate definition of the impurity Hamiltonian for such systems
is presented and demonstrated in cases of 1, 2 and 3 dimensions, using coupled
cluster theory as the impurity solver. Additionally, we discuss the challenges
related to disentanglement of fragment and bath states. The current approach
yields results comparable to coupled cluster calculations of infinite systems
even when using a single unit cell as the fragment. The theory is formulated in
the basis of Wannier functions but it does not require separate localization of
unoccupied bands. The embedding scheme presented here is a promising way of
employing highly accurate electronic structure methods for extended systems at
a fraction of their original computational cost
High-density correlation energy expansion of the one-dimensional uniform electron gas
We show that the expression of the high-density (i.e small-) correlation
energy per electron for the one-dimensional uniform electron gas can be
obtained by conventional perturbation theory and is of the form \Ec(r_s) =
-\pi^2/360 + 0.00845 r_s + ..., where is the average radius of an
electron. Combining these new results with the low-density correlation energy
expansion, we propose a local-density approximation correlation functional,
which deviates by a maximum of 0.1 millihartree compared to the benchmark DMC
calculations.Comment: 7 pages, 2 figures, 3 tables, accepted for publication in J. Chem.
Phy
Exchange parameters from approximate self-interaction correction scheme
The approximate atomic self-interaction corrections (ASIC) method to density
functional theory is put to the test by calculating the exchange interaction
for a number of prototypical materials, critical to local exchange and
correlation functionals. ASIC total energy calculations are mapped onto an
Heisenberg pair-wise interaction and the exchange constants J are compared to
those obtained with other methods. In general the ASIC scheme drastically
improves the bandstructure, which for almost all the cases investigated
resemble closely available photo-emission data. In contrast the results for the
exchange parameters are less satisfactory. Although ASIC performs reasonably
well for systems where the magnetism originates from half-filled bands, it
suffers from similar problems than those of LDA for other situations. In
particular the exchange constants are still overestimated. This reflects a
subtle interplay between exchange and correlation energy, not captured by the
ASIC.Comment: 10 page
Assembly and analysis of fragmentation data for liquid propellant vessels
Fragmentation data was assembled and analyzed for exploding liquid propellant vessels. These data were to be retrieved from reports of tests and accidents, including measurements or estimates of blast yield, etc. A significant amount of data was retrieved from a series of tests conducted for measurement of blast and fireball effects of liquid propellant explosions (Project PYRO), a few well-documented accident reports, and a series of tests to determine auto-ignition properties of mixing liquid propellants. The data were reduced and fitted to various statistical functions. Comparisons were made with methods of prediction for blast yield, initial fragment velocities, and fragment range. Reasonably good correlation was achieved. Methods presented in the report allow prediction of fragment patterns, given type and quantity of propellant, type of accident, and time of propellant mixing
Voltage-Controlled Surface Magnetization of Itinerant Ferromagnet Ni_(1-x)Cu_x
We argue that surface magnetization of a metallic ferromagnet can be turned
on and off isothermally by an applied voltage. For this, the material's
electron subsystem must be close enough to the boundary between para- and
ferromagnetic regions on the electron density scale. For the 3d series, the
boundary is between Ni and Cu, which makes their alloy a primary candidate.
Using Ginzburg-Landau functional, which we build from Ni_(1-x)Cu_x empirical
properties, ab-initio parameters of Ni and Cu, and orbital-free LSDA, we show
that the proposed effect is experimentally observable.Comment: 4 pages; 2 figures; submitted to PRL February 16th 2008; transferred
to PRB June 21st 2008; published July 15th 200
Toward transferable interatomic van der Waals interactions without electrons: The role of multipole electrostatics and many-body dispersion
We estimate polarizabilities of atoms in molecules without electron density,
using a Voronoi tesselation approach instead of conventional density
partitioning schemes. The resulting atomic dispersion coefficients are
calculated, as well as many-body dispersion effects on intermolecular potential
energies. We also estimate contributions from multipole electrostatics and
compare them to dispersion. We assess the performance of the resulting
intermolecular interaction model from dispersion and electrostatics for more
than 1,300 neutral and charged, small organic molecular dimers. Applications to
water clusters, the benzene crystal, the anti-cancer drug
ellipticine---intercalated between two Watson-Crick DNA base pairs, as well as
six macro-molecular host-guest complexes highlight the potential of this method
and help to identify points of future improvement. The mean absolute error made
by the combination of static electrostatics with many-body dispersion reduces
at larger distances, while it plateaus for two-body dispersion, in conflict
with the common assumption that the simple correction will yield proper
dissociative tails. Overall, the method achieves an accuracy well within
conventional molecular force fields while exhibiting a simple parametrization
protocol.Comment: 13 pages, 8 figure
Hooke's law correlation in two-electron systems
We study the properties of the Hooke's law correlation energy (\Ec),
defined as the correlation energy when two electrons interact {\em via} a
harmonic potential in a -dimensional space. More precisely, we investigate
the ground state properties of two model systems: the Moshinsky atom (in
which the electrons move in a quadratic potential) and the spherium model (in
which they move on the surface of a sphere). A comparison with their Coulombic
counterparts is made, which highlights the main differences of the \Ec in
both the weakly and strongly correlated limits. Moreover, we show that the
Schr\"odinger equation of the spherium model is exactly solvable for two values
of the dimension (), and that the exact wave function is
based on Mathieu functions.Comment: 7 pages, 5 figure
Density-density functionals and effective potentials in many-body electronic structure calculations
We demonstrate the existence of different density-density functionals
designed to retain selected properties of the many-body ground state in a
non-interacting solution starting from the standard density functional theory
ground state. We focus on diffusion quantum Monte Carlo applications that
require trial wave functions with optimal Fermion nodes. The theory is
extensible and can be used to understand current practices in several
electronic structure methods within a generalized density functional framework.
The theory justifies and stimulates the search of optimal empirical density
functionals and effective potentials for accurate calculations of the
properties of real materials, but also cautions on the limits of their
applicability. The concepts are tested and validated with a near-analytic
model.Comment: five figure
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