496 research outputs found
Double-Pole Approximation in Time-Dependent Density Functional Theory
A simple approximate solution to the linear response equations of
time-dependent density functional theory (TDDFT) is given. This extends the
single-pole approximation (SPA) to two strongly-coupled poles. The analysis
provides both an illustration of how TDDFT works when strong
exchange-correlation effects are present and insight into such corrections. For
example, interaction can cause a transition to vanish entirely from the optical
spectrum.Comment: 7 pages, 11 figure
Correlated non-perturbative electron dynamics with quantum trajectories
An approach to electron correlation effects in atoms that uses quantum
trajectories is presented. A comparison with the exact quantum mechanical
results for 1D Helium atom shows that the major features of the correlated
ground state distribution and of the strong field ionization dynamics are
reproduced with quantum trajectories. The intra-atomic resonant transitions are
described accurately by a trajectory ensemble. The present approach reduces
significantly the computational time and it can be used for both bound and
ionizing electrons.Comment: 9 pages, 4 figure
Momentum distributions in time-dependent density functional theory: Product phase approximation for non-sequential double ionization in strong laser fields
We investigate the possibility to deduce momentum space properties from
time-dependent density functional calculations. Electron and ion momentum
distributions after double ionization of a model Helium atom in a strong
few-cycle laser pulse are studied. We show that, in this case, the choice of
suitable functionals for the observables is considerably more important than
the choice of the correlation potential in the time-dependent Kohn-Sham
equations. By comparison with the solution of the time-dependent Schroedinger
equation, the insufficiency of functionals neglecting electron correlation is
demonstrated. We construct a functional of the Kohn-Sham orbitals, which in
principle yields the exact momentum distributions of the electrons and the ion.
The product-phase approximation is introduced, which reduces the problem of
approximating this functional significantly.Comment: 8 pages, 5 figures, RevTeX
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
The generator coordinate method in time-dependent density-functional theory: memory made simple
The generator coordinate (GC) method is a variational approach to the quantum
many-body problem in which interacting many-body wave functions are constructed
as superpositions of (generally nonorthogonal) eigenstates of auxiliary
Hamiltonians containing a deformation parameter. This paper presents a
time-dependent extension of the GC method as a new approach to improve existing
approximations of the exchange-correlation (XC) potential in time-dependent
density-functional theory (TDDFT). The time-dependent GC method is shown to be
a conceptually and computationally simple tool to build memory effects into any
existing adiabatic XC potential. As an illustration, the method is applied to
driven parametric oscillations of two interacting electrons in a harmonic
potential (Hooke's atom). It is demonstrated that a proper choice of
time-dependent generator coordinates in conjunction with the adiabatic
local-density approximation reproduces the exact linear and nonlinear
two-electron dynamics quite accurately, including features associated with
double excitations that cannot be captured by TDDFT in the adiabatic
approximation.Comment: 10 pages, 13 figure
Time-dependent density functional theory for strong electromagnetic fields in crystalline solids
We apply the coupled dynamics of time-dependent density functional theory and
Maxwell equations to the interaction of intense laser pulses with crystalline
silicon. As a function of electromagnetic field intensity, we see several
regions in the response. At the lowest intensities, the pulse is reflected and
transmitted in accord with the dielectric response, and the characteristics of
the energy deposition is consistent with two-photon absorption. The absorption
process begins to deviate from that at laser intensities ~ 10^13 W/cm^2, where
the energy deposited is of the order of 1 eV per atom. Changes in the
reflectivity are seen as a function of intensity. When it passes a threshold of
about 3 \times 1012 W/cm2, there is a small decrease. At higher intensities,
above 2 \times 10^13 W/cm^2, the reflectivity increases strongly. This behavior
can be understood qualitatively in a model treating the excited electron-hole
pairs as a plasma.Comment: 27 pages; 11 figure
The correlation potential in density functional theory at the GW-level: spherical atoms
As part of a project to obtain better optical response functions for nano
materials and other systems with strong excitonic effects we here calculate the
exchange-correlation (XC) potential of density-functional theory (DFT) at a
level of approximation which corresponds to the dynamically- screened-exchange
or GW approximation. In this process we have designed a new numerical method
based on cubic splines which appears to be superior to other techniques
previously applied to the "inverse engineering problem" of DFT, i.e., the
problem of finding an XC potential from a known particle density. The
potentials we obtain do not suffer from unphysical ripple and have, to within a
reasonable accuracy, the correct asymptotic tails outside localized systems.
The XC potential is an important ingredient in finding the particle-conserving
excitation energies in atoms and molecules and our potentials perform better in
this regard as compared to the LDA potential, potentials from GGA:s, and a DFT
potential based on MP2 theory.Comment: 13 pages, 9 figure
Time-dependent density functional theory: Past, present, and future
Time-dependent density functional theory (TDDFT) is presently enjoying
enormous popularity in quantum chemistry, as a useful tool for extracting
electronic excited state energies. This article discusses how TDDFT is much
broader in scope, and yields predictions for many more properties. We discuss
some of the challenges involved in making accurate predictions for these
properties.Comment: 12 pages, 4 figure
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