5,602 research outputs found
Ultra-nonlocality in density functional theory for photo-emission spectroscopy
We derive an exact expression for the photo-current of photo-emission
spectroscopy using time-dependent current density functional theory (TDCDFT).
This expression is given as an integral over the Kohn-Sham spectral function
renormalized by effective potentials that depend on the exchange-correlation
kernel of current density functional theory. We analyze in detail the physical
content of this expression by making a connection between the
density-functional expression and the diagrammatic expansion of the
photo-current within many-body perturbation theory. We further demonstrate that
the density functional expression does not provide us with information on the
kinetic energy distribution of the photo-electrons. Such information can, in
principle, be obtained from TDCDFT by exactly modeling the experiment in which
the photo-current is split into energy contributions by means of an external
electromagnetic field outside the sample, as is done in standard detectors. We
find, however, that this procedure produces very nonlocal correlations between
the exchange-correlation fields in the sample and the detector.Comment: 11 pages, 11 figure
A systematic benchmark of the ab initio Bethe-Salpeter equation approach for low-lying optical excitations of small organic molecules
The predictive power of the ab initio Bethe-Salpeter equation (BSE) approach,
rigorously based on many-body Green's function theory but incorporating
information from density functional theory, has already been demonstrated for
the optical gaps and spectra of solid-state systems. Interest in photoactive
hybrid organic/inorganic systems has recently increased, and so has the use of
the BSE for computing neutral excitations of organic molecules. However, no
systematic benchmarks of the BSE for neutral electronic excitations of organic
molecules exist. Here, we study the performance of the BSE for the 28 small
molecules in Thiel's widely-used time-dependent density functional theory
benchmark set [M. Schreiber et al. J. Chem. Phys. 128, 134110 (2008)]. We
observe that the BSE produces results that depend critically on the mean-field
starting point employed in the perturbative approach. We find that this
starting point dependence is mainly introduced through the quasiparticle
energies obtained at the intermediate GW step, and that with a judicious choice
of starting mean-field, singlet excitation energies obtained from BSE are in
excellent quantitative agreement with higher-level wavefunction methods. The
quality of the triplet excitations is slightly less satisfactory
GAS6 expressing and self-sustaining cancer cells in 3D spheroids activate the PDK-RSK-mTOR pathway for survival and drug resistance.
AXL Receptor Tyrosine Kinase (RTK) inhibition presents a promising therapeutic strategy for aggressive tumor subtypes, as AXL signaling is upregulated in many cancers resistant to first line treatments. Furthermore, the AXL ligand Growth Arrest-Specific gene 6 (GAS6) has recently been linked to cancer drug resistance. Here, we established that challenging conditions, such as serum deprivation, divide AXL overexpressing tumor cell lines into non-self-sustaining and self-sustaining subtypes in 3D spheroid culture. Self-sustaining cells are characterized by excessive GAS6 secretion and TAM-PDK-RSK-mTOR pathway activation. In 3D spheroid culture, the activation of the TAM-PDK-RSK-mTOR pathway proves crucial following treatment with AXL/MET inhibitor BMS777607, when the self-sustaining tumor cells react with TAM-RSK hyper-activation and enhanced SRC-AKT-mTOR signaling. Thus, bidirectional activated mTOR leads to enhanced proliferation and counteracts the drug effect. mTOR activation is accompanied by an enhanced AXL expression and hyper-phosphorylation following 24 hours of treatment with BMS777607. Thereby, we elucidate a double role of AXL which can be assigned to RSK-mTOR as well as SRC-AKT-mTOR pathway activation, specifically through AXL Y779 phosphorylation. This phosphosite fuels the resistance mechanism in 3D spheroids, alongside further SRC dependent EGFR Y1173 and/or MET Y1349 phosphorylation which is defined by the cell-specific addiction. In conclusion, self-sustenance in cancer cells is based on a signaling synergy, individually balanced between GAS6 TAM dependent PDK-RSK-mTOR survival pathway and the AXLY779/EGFR/MET driven SRC-mTOR pathway. Molecular Oncology (2017) 2017 The Authors. Published by FEBS Press and John Wiley & Sons Ltd
Excitations and benchmark ensemble density functional theory for two electrons
A new method for extracting ensemble Kohn-Sham potentials from accurate
excited state densities is applied to a variety of two electron systems,
exploring the behavior of exact ensemble density functional theory. The issue
of separating the Hartree energy and the choice of degenerate eigenstates is
explored. A new approximation, spin eigenstate Hartree-exchange (SEHX), is
derived. Exact conditions that are proven include the signs of the correlation
energy components, the virial theorem for both exchange and correlation, and
the asymptotic behavior of the potential for small weights of the excited
states. Many energy components are given as a function of the weights for two
electrons in a one-dimensional flat box, in a box with a large barrier to
create charge transfer excitations, in a three-dimensional harmonic well
(Hooke's atom), and for the He atom singlet-triplet ensemble,
singlet-triplet-singlet ensemble, and triplet bi-ensemble.Comment: 15 pages, supplemental material pd
A novel method for unambiguous ion identification in mixed ion beams extracted from an EBIT
A novel technique to identify small fluxes of mixed highly charged ion beams
extracted from an Electron Beam Ion Trap (EBIT) is presented and practically
demonstrated. The method exploits projectile charge state dependent potential
emission of electrons as induced by ion impact on a metal surface to separate
ions with identical or very similar mass-to-charge ratio.Comment: 8 pages, 5 figure
Semiclassical model for calculating fully differential ionization cross sections of the H molecule
Fully differential cross sections are calculated for the ionization of H
by fast charged projectiles using a semiclassical model developed previously
for the ionization of atoms. The method is tested in case of 4 keV electron and
6 MeV proton projectiles. The obtained results show good agreement with the
available experimental data. Interference effects due to the two-center
character of the target are also observed and analyzed.Comment: 11 pages, 4 figure
Degenerate ground states and nonunique potentials: breakdown and restoration of density functionals
The Hohenberg-Kohn (HK) theorem is one of the most fundamental theorems of
quantum mechanics, and constitutes the basis for the very successful
density-functional approach to inhomogeneous interacting many-particle systems.
Here we show that in formulations of density-functional theory (DFT) that
employ more than one density variable, applied to systems with a degenerate
ground state, there is a subtle loophole in the HK theorem, as all mappings
between densities, wave functions and potentials can break down. Two weaker
theorems which we prove here, the joint-degeneracy theorem and the
internal-energy theorem, restore the internal, total and exchange-correlation
energy functionals to the extent needed in applications of DFT to atomic,
molecular and solid-state physics and quantum chemistry. The joint-degeneracy
theorem constrains the nature of possible degeneracies in general many-body
systems
Dynamic Properties of Fine-Grained Coal Refuse
A simplified method of slope stability analysis is presented for upstream-constructed coal tailings dams subjected to earthquake shaking. The method employs a conventional method of slices approach, in which dynamic loads are represented as psuedostatic forces applied to each slice. Excess pore water pressures are estimated from cyclic triaxial tests performed on specimens of fine coal refuse. Cyclic triaxial test results are presented for fine coal refuse materials from six sites in the western Appalachian region. Measured excess pore water pressure values appear to be influenced by fine coal processing procedures, although material plasticity and grain characteristics are also important. Additional studies are ongoing
Spin gaps and spin-flip energies in density-functional theory
Energy gaps are crucial aspects of the electronic structure of finite and
extended systems. Whereas much is known about how to define and calculate
charge gaps in density-functional theory (DFT), and about the relation between
these gaps and derivative discontinuities of the exchange-correlation
functional, much less is know about spin gaps. In this paper we give
density-functional definitions of spin-conserving gaps, spin-flip gaps and the
spin stiffness in terms of many-body energies and in terms of single-particle
(Kohn-Sham) energies. Our definitions are as analogous as possible to those
commonly made in the charge case, but important differences between spin and
charge gaps emerge already on the single-particle level because unlike the
fundamental charge gap spin gaps involve excited-state energies. Kohn-Sham and
many-body spin gaps are predicted to differ, and the difference is related to
derivative discontinuities that are similar to, but distinct from, those
usually considered in the case of charge gaps. Both ensemble DFT and
time-dependent DFT (TDDFT) can be used to calculate these spin discontinuities
from a suitable functional. We illustrate our findings by evaluating our
definitions for the Lithium atom, for which we calculate spin gaps and spin
discontinuities by making use of near-exact Kohn-Sham eigenvalues and,
independently, from the single-pole approximation to TDDFT. The many-body
corrections to the Kohn-Sham spin gaps are found to be negative, i.e., single
particle calculations tend to overestimate spin gaps while they underestimate
charge gaps.Comment: 11 pages, 1 figure, 3 table
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