3,638 research outputs found
Comment on "Modifying the variational principle in the action integral functional derivation of time-dependent density functional theory" by Jochen Schirmer [arXiv:1010.4223]
In a paper recently published in Phys. Rev. A [arXiv:1010.4223], Schirmer has
criticized an earlier work of mine [arXiv:0803.2727], as well as the
foundations of time-dependent density functional theory. In Ref.[2], I showed
that the so-called "causality paradox" - i.e., the failure of the
exchange-correlation potential derived from the Runge-Gross time-dependent
variational principle to satisfy causality requirements - can be solved by a
careful reformulation of that variational principle. Fortunately, the criticism
presented in Ref.[1] is based on elementary misunderstandings of the nature of
functionals, gauge transformations, and the time-dependent variational
principle. In this Comment I wish to point out and clear these
misunderstandings.Comment: 4 pages. Accepted for publication in Phys. Rev.
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
Closed-form expressions for correlated density matrices: application to dispersive interactions and example of (He)2
Empirically correlated density matrices of N-electron systems are
investigated. Exact closed-form expressions are derived for the one- and
two-electron reduced density matrices from a general pairwise correlated wave
function. Approximate expressions are proposed which reflect dispersive
interactions between closed-shell centro-symmetric subsystems. Said expressions
clearly illustrate the consequences of second-order correlation effects on the
reduced density matrices. Application is made to a simple example: the (He)2
system. Reduced density matrices are explicitly calculated, correct to second
order in correlation, and compared with approximations of independent electrons
and independent electron pairs. The models proposed allow for variational
calculations of interaction energies and equilibrium distance as well as a
clear interpretation of dispersive effects on electron distributions. Both
exchange and second order correlation effects are shown to play a critical role
on the quality of the results.Comment: 22 page
Hole polaron formation and migration in olivine phosphate materials
By combining first principles calculations and experimental XPS measurements,
we investigate the electronic structure of potential Li-ion battery cathode
materials LiMPO4 (M=Mn,Fe,Co,Ni) to uncover the underlying mechanisms that
determine small hole polaron formation and migration. We show that small hole
polaron formation depends on features in the electronic structure near the
valence-band maximum and that, calculationally, these features depend on the
methodology chosen for dealing with the correlated nature of the
transition-metal d-derived states in these systems. Comparison with experiment
reveals that a hybrid functional approach is superior to GGA+U in correctly
reproducing the XPS spectra. Using this approach we find that LiNiPO4 cannot
support small hole polarons, but that the other three compounds can. The
migration barrier is determined mainly by the strong or weak bonding nature of
the states at the top of the valence band, resulting in a substantially higher
barrier for LiMnPO4 than for LiCoPO4 or LiFePO4
Interactions and Broken Time-Reversal Symmetry in Chaotic Quantum Dots
When treating interactions in quantum dots within a RPA-like approach,
time-reversal symmetry plays an important role as higher-order terms -- the
Cooper series -- need to be included when this symmetry is present. Here we
consider model quantum dots in a magnetic field weak enough to leave the
dynamics of the dot chaotic, but strong enough to break time-reversal symmetry.
The ground state spin and addition energy for dots containing 120 to 200
electrons are found using local spin density functional theory, and we compare
the corresponding distributions with those derived from an RPA-like treatment
of the interactions. The agreement between the two approaches is very good,
significantly better than for analogous calculations in the presence of
time-reversal symmetry. This demonstrates that the discrepancies between the
two approaches in the time-reversal symmetric case indeed originate from the
Cooper channel, indicating that these higher-order terms might not be properly
taken into account in the spin density functional calculations.Comment: 4 pages, 3 figure
Ground state of two electrons on concentric spheres
We extend our analysis of two electrons on a sphere [Phys. Rev. A {\bf 79},
062517 (2009); Phys. Rev. Lett. {\bf 103}, 123008 (2009)] to electrons on
concentric spheres with different radii. The strengths and weaknesses of
several electronic structure models are analyzed, ranging from the mean-field
approximation (restricted and unrestricted Hartree-Fock solutions) to
configuration interaction expansion, leading to near-exact wave functions and
energies. The M{\o}ller-Plesset energy corrections (up to third-order) and the
asymptotic expansion for the large-spheres regime are also considered. We also
study the position intracules derived from approximate and exact wave
functions. We find evidence for the existence of a long-range Coulomb hole in
the large-spheres regime, and infer that unrestricted Hartree-Fock theory
over-localizes the electrons.Comment: 10 pages, 10 figure
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