156 research outputs found
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
Accurate density functional calculations on frequency-dependent hyperpolarizabilities of small molecules
In this paper we present time-dependent density functional calculations on frequency-dependent first (β) and second (γ) hyperpolarizabilities for the set of small molecules,
Time-Dependent Density Functional Theory of Open Quantum Systems in the Linear-Response Regime
Time-Dependent Density Functional Theory (TDDFT) has recently been extended
to describe many-body open quantum systems (OQS) evolving under non-unitary
dynamics according to a quantum master equation. In the master equation
approach, electronic excitation spectra are broadened and shifted due to
relaxation and dephasing of the electronic degrees of freedom by the
surrounding environment. In this paper, we develop a formulation of TDDFT
linear-response theory (LR-TDDFT) for many-body electronic systems evolving
under a master equation, yielding broadened excitation spectra. This is done by
mapping an interacting open quantum system onto a non-interacting open
Kohn-Sham system yielding the correct non-equilibrium density evolution. A
pseudo-eigenvalue equation analogous to the Casida equations of usual LR-TDDFT
is derived for the Redfield master equation, yielding complex energies and Lamb
shifts. As a simple demonstration, we calculate the spectrum of a C atom
in an optical resonator interacting with a bath of photons. The performance of
an adiabatic exchange-correlation kernel is analyzed and a first-order
frequency-dependent correction to the bare Kohn-Sham linewidth based on
Gorling-Levy perturbation theory is calculated.Comment: 18 pages, 4 figure
Conserving Approximations in Time-Dependent Density Functional Theory
In the present work we propose a theory for obtaining successively better
approximations to the linear response functions of time-dependent density or
current-density functional theory. The new technique is based on the
variational approach to many-body perturbation theory (MBPT) as developed
during the sixties and later expanded by us in the mid nineties. Due to this
feature the resulting response functions obey a large number of conservation
laws such as particle and momentum conservation and sum rules. The quality of
the obtained results is governed by the physical processes built in through
MBPT but also by the choice of variational expressions. We here present several
conserving response functions of different sophistication to be used in the
calculation of the optical response of solids and nano-scale systems.Comment: 11 pages, 4 figures, revised versio
Density-functional calculation of ionization energies of current-carrying atomic states
Current-density-functional theory is used to calculate ionization energies of
current-carrying atomic states. A perturbative approximation to full
current-density-functional theory is implemented for the first time, and found
to be numerically feasible. Different parametrizations for the
current-dependence of the density functional are critically compared. Orbital
currents in open-shell atoms turn out to produce a small shift in the
ionization energies. We find that modern density functionals have reached an
accuracy at which small current-related terms appearing in open-shell
configurations are not negligible anymore compared to the remaining difference
to experiment.Comment: 7 pages, 2 tables, accepted by Phys. Rev.
Many-body diagrammatic expansion in a Kohn-Sham basis: implications for Time-Dependent Density Functional Theory of excited states
We formulate diagrammatic rules for many-body perturbation theory which uses
Kohn-Sham (KS) Green's functions as basic propagators. The diagram technique
allows to study the properties of the dynamic nonlocal exchange-correlation
(xc) kernel . We show that the spatial non-locality of is
strongly frequency-dependent. In particular, in extended systems the
non-locality range diverges at the excitation energies. This divergency is
related to the discontinuity of the xc potential.Comment: 4 RevTeX pages including 3 eps figures, submitted to Phys. Rev. Lett;
revised version with new reference
Plasmon Lifetime in K: A Case Study of Correlated Electrons in Solids Amenable to Ab Initio Theory
On the basis of a new ab initio, all-electron response scheme, formulated
within time-dependent density-functional theory, we solve the puzzle posed by
the anomalous dispersion of the plasmon linewidth in K. The key damping
mechanism is shown to be decay into particle-hole pairs involving empty states
of d-symmetry. While the effect of many-particle correlations is small, the
correlations built into the "final-state" -d-bands play an important, and
novel, role ---which is related to the phase-space complexity associated with
these flat bands. Our case study of plasmon lifetime in K illustrates the
importance of ab initio paradigms for the study of excitations in
correlated-electron systems.Comment: 12 pages, 4 figures, for html browsing see http://web.utk.edu/~weik
C in intense femtosecond laser pulses: nonlinear dipole response and ionization
We study the interaction of strong femtosecond laser pulses with the C
molecule employing time-dependent density functional theory with the ionic
background treated in a jellium approximation. The laser intensities considered
are below the threshold of strong fragmentation but too high for perturbative
treatments such as linear response. The nonlinear response of the model to
excitations by short pulses of frequencies up to 45eV is presented and analyzed
with the help of Kohn-Sham orbital resolved dipole spectra. In femtosecond
laser pulses of 800nm wavelength ionization is found to occur multiphoton-like
rather than via excitation of a ``giant'' resonance.Comment: 14 pages, including 1 table, 5 figure
Current-Density Functional Theory of the Response of Solids
The response of an extended periodic system to a homogeneous field (of
wave-vector ) cannot be obtained from a time-dependent density
functional theory (TDDFT) calculation, because the
Runge-Gross theorem does not apply. Time-dependent {\em current}-density
functional theory is needed and demonstrates that one key ingredient missing
from TDDFT is the macroscopic current. In the low-frequency limit, in certain
cases, density polarization functional theory is recovered and a formally exact
expression for the polarization functional is given.Comment: 5 pages, accepted in PR
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