86 research outputs found
Undoing static correlation: Long-range charge transfer in time-dependent density functional theory
Long-range charge transfer excited states are notoriously badly
underestimated in time-dependent density functional theory (TDDFT). We resolve
how {\it exact} TDDFT captures charge transfer between open-shell species: in
particular the role of the step in the ground-state potential, and the severe
frequency-dependence of the exchange-correlation kernel. An expression for the
latter is derived, that becomes exact in the limit that the charge-transfer
excitations are well-separated from other excitations. The exchange-correlation
kernel has the task of undoing the static correlation in the ground state
introduced by the step, in order to accurately recover the physical
charge-transfer states.Comment: 2 figure
Novel properties of the Kohn-Sham exchange potential for open systems: application to the two-dimensional electron gas
The properties of the Kohn-Sham (KS) exchange potential for open systems in
thermodynamical equilibrium, where the number of particles is non-conserved,
are analyzed with the Optimized Effective Potential (OEP) method of Density
Functional Theory (DFT) at zero temperature. The quasi two-dimensional electron
gas (2DEG) is used as an illustrative example. The main findings are that the
KS exchange potential builds a significant barrier-like structure under slight
population of the second subband, and that both the asymptotic value of the KS
exchange potential and the inter-subband energy jump discontinuously at the
one-subband (1S) -> two-subband (2S) transition. The results obtained in this
system offer new insights on open problems of semiconductors, such as the
band-gap underestimation and the band-gap renormalization by photo-excited
carriers.Comment: 7 pages, 3 figures, uses epl.cls(included), accepted for publication
in Europhysics Letter
A Paradigm of Translational Medicine
Antimicrobial peptides (AMPs) are small, cationic, amphiphilic peptides with
broad-spectrum microbicidal activity against both bacteria and fungi. In
mammals, AMPs form the first line of host defense against infections and
generally play an important role as effector agents of the innate immune
system. The AMP era was born more than 6 decades ago when the first cationic
cyclic peptide antibiotics, namely polymyxins and tyrothricin, found their way
into clinical use. Due to the good clinical experience in the treatment of,
for example, infections of mucus membranes as well as the subsequent
understanding of mode of action, AMPs are now considered for treatment of
inflammatory skin diseases and for improving healing of infected wounds. Based
on the preclinical findings, including pathobiochemistry and molecular
medicine, targeted therapy strategies are developed and first results indicate
that AMPs influence processes of diseased skin. Importantly, in contrast to
other antibiotics, AMPs do not seem to propagate the development of
antibiotic-resistant micro-organisms. Therefore, AMPs should be tested in
clinical trials for their efficacy and tolerability in inflammatory skin
diseases and chronic wounds. Apart from possible fields of application, these
peptides appear suited as an example of the paradigm of translational medicine
for skin diseases which is today seen as a ‘two-way road’ – from bench to
bedside and backwards from bedside to bench
Becke-Johnson-type exchange potential for two-dimensional systems
We extend the Becke-Johnson approximation [J. Chem. Phys. 124, 221101 (2006)]
of the exchange potential to two dimensions. We prove and demonstrate that a
direct extension of the underlying formalism may lead to divergent behavior of
the potential. We derive a cure to the approach by enforcing the gauge
invariance and correct asymptotic behavior of the exchange potential. The
procedure leads to an approximation which is shown, in various
quasi-two-dimensional test systems, to be very accurate in comparison with the
exact exchange potential, and thus a considerable improvement over the commonly
applied local-density approximation.Comment: submitted to Phys. Rev. B on July 9th, 200
Violation of the `Zero-Force Theorem' in the time-dependent Krieger-Li-Iafrate approximation
We demonstrate that the time-dependent Krieger-Li-Iafrate approximation in
combination with the exchange-only functional violates the `Zero-Force
Theorem'. By analyzing the time-dependent dipole moment of Na5 and Na9+, we
furthermore show that this can lead to an unphysical self-excitation of the
system depending on the system properties and the excitation strength.
Analytical aspects, especially the connection between the `Zero-Force Theorem'
and the `Generalized-Translation Invariance' of the potential, are discussed.Comment: 5 pages, 4 figure
Orbital densities functional
Local density approximation (LDA) to the density functional theory (DFT) has
continuous derivative of total energy as a number of electrons function and
continuous exchange-correlation potential, while in exact DFT both should be
discontinuous as number of electrons goes through an integer value. We propose
orbital densities functional (ODF) (with orbitals defined as Wannier functions)
that by construction obeys this discontinuity condition. By its variation
one-electron equations are obtained with potential in the form of projection
operator. The operator increases a separation between occupied and empty bands
thus curing LDA deficiency of energy gap value systematic underestimation.
Orbital densities functional minimization gives ground state orbital and total
electron densities. The ODF expression for the energy of orbital densities
fluctuations around the ground state values defines ODF fluctuation Hamiltonian
that allows to treat correlation effects. Dynamical mean-field theory (DMFT)
was used to solve this Hamiltonian with quantum Monte Carlo (QMC) method for
effective impurity problem. We have applied ODF method to the problem of
metal-insulator transition in lanthanum trihydride LaH_{3-x}. In LDA
calculations ground state of this material is metallic for all values of
hydrogen nonstoichiometry x while experimentally the system is insulating for x
< 0.3. ODF method gave paramagnetic insulator solution for LaH_3 and LaH_{2.75}
but metallic state for LaH_{2.5}.Comment: 35 pages, 5 figure
Exact-exchange Kohn-Sham potential, surface energy, and work function of jellium slabs
Exact-exchange self-consistent calculations of the Kohn-Sham potential,
surface energy, and work function of jellium slabs are reported in the
framework of the Optimized Effective Potential (OEP) scheme of Density
Functional Theory. In the vacuum side of the jellium surface and at a distance
that is larger than the slab thickness, the exchange-only Kohn-Sham
potential is found to be image-like () but with a coefficient that
differs from that of the classical image potential . The
three OEP contributions to the surface energy (kinetic, electrostatic, and
exchange) are found to oscillate as a function of the slab thickness, as occurs
in the case of the corresponding calculations based on the use of
single-particle orbitals and energies obtained in the Local Density
Approximation (LDA). The OEP work function presents large quantum size effects
that are absent in the LDA and which reflect the intrinsic derivative
discontinuity of the exact Kohn-Sham potential.Comment: 25 pages, 9 figures, to appear in Phys. Rev.
Parameterized optimized effective potential for atoms
The optimized effective potential equations for atoms have been solved by
parameterizing the potential. The expansion is tailored to fulfill the known
asymptotic behavior of the effective potential at both short and long
distances. Both single configuration and multi configuration trial wave
functions are implemented. Applications to several atomic systems are presented
improving previous works. The results here obtained are very close to those
calculated in either the Hartree-Fock and the multi configurational
Hartree-Fock framework.Comment: 8 pages, 3 figure
Ab initio Green's function formalism for band structures
Using the Green's function formalism, an ab initio theory for band structures
of crystals is derived starting from the Hartree-Fock approximation. It is
based on the algebraic diagrammatic construction scheme for the self-energy
which is formulated for crystal orbitals (CO-ADC). In this approach, the poles
of the Green's function are determined by solving a suitable Hermitian
eigenvalue problem. The method is not only applicable to the outer valence and
conduction bands, it is also stable for inner valence bands where strong
electron correlations are effective. The key to the proposed scheme is to
evaluate the self-energy in terms of Wannier orbitals before transforming it to
a crystal momentum representation. Exploiting the fact that electron
correlations are mainly local, one can truncate the lattice summations by an
appropriate configuration selection scheme. This yields a flat configuration
space; i.e., its size scales only linearly with the number of atoms per unit
cell for large systems and, under certain conditions, the computational effort
to determine band structures also scales linearly. As a first application of
the new formalism, a lithium fluoride crystal has been chosen. A minimal basis
set description is studied, and a satisfactory agreement with previous
theoretical and experimental results for the fundamental band gap and the width
of the F 2p valence band complex is obtained.Comment: 20 pages, 3 figures, 1 table, RevTeX4, new section on lithium
fluorid
Density-functional Study of Small Molecules within the Krieger-Li-Iafrate Approximation
We report density-functional studies of several small molecules (, and ) within the Krieger-Li-Iafrate (KLI)
approximation to the exact Kohn-Sham local exchange potential, using a
three-dimensional real-space finite-difference pseudopotential method. It is
found that exchange-only KLI leads to markedly improved eigenvalue spectra
compared to those obtained within the standard local-density approximation
(LDA), the generalized gradient approximation (GGA), and the Hartree-Fock (HF)
method. For structural properties, exchange-only KLI results are close to the
corresponding HF values. We find that the addition of LDA or GGA correlation
energy functionals to the KLI exact exchange energy functional does not lead to
systematic improvements.Comment: 16 pages including 1 fugure, to be published in Phys. Rev. A Nov. 1
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