945 research outputs found
Alloying effects on the optical properties of GeSi nanocrystals from TDDFT and comparison with effective-medium theory
We present the optical spectra of GeSi alloy nanocrystals
calculated with time-dependent density-functional theory in the adiabatic
local-density ap proximation (TDLDA). The spectra change smoothly as a function
of the compositio n . On the Ge side of the composition range, the lowest
excitations at the ab sorption edge are almost pure Kohn-Sham
independent-particle HOMO-LUMO transitio ns, while for higher Si contents
strong mixing of transitions is found. Within T DLDA the first peak is slightly
higher in energy than in earlier independent-par ticle calculations. However,
the absorption onset and in particular its composit ion dependence is similar
to independent-particle results. Moreover, classical depolarization effects are
responsible for a very strong suppression of the abs orption intensity. We show
that they can be taken into account in a simpler way using Maxwell-Garnett
classical effective-medium theory. Emission spectra are in vestigated by
calculating the absorption of excited nanocrystals at their relaxe d geometry.
The structural contribution to the Stokes shift is about 0.5 eV. Th e
decomposition of the emission spectra in terms of independent-particle transit
ions is similar to what is found for absorption. For the emission, very weak
tra nsitions are found in Ge-rich clusters well below the strong absorption
onset.Comment: submitted to Phys. Rev.
Troubleshooting Time-Dependent Density-Functional Theory for Photochemical Applications: Oxirane
The development of analytic-gradient methodology for excited states within
conventional time-dependent density-functional theory (TDDFT) would seem to
offer a relatively inexpensive alternative to better established
quantum-chemical approaches for the modeling of photochemical reactions.
However, even though TDDFT is formally exact, practical calculations involve
the use of approximate functionals, in particular the TDDFT adiabatic
approximation, whose use in photochemical applications must be further
validated. Here, we investigate the prototypical case of the symmetric CC ring
opening of oxirane. We demonstrate by direct comparison with the results of
high-quality quantum Monte Carlo calculations that, far from being an
approximation on TDDFT, the Tamm-Dancoff approximation (TDA) is a practical
necessity for avoiding triplet instabilities and singlet near instabilities,
thus helping maintain energetically reasonable excited-state potential energy
surfaces during bond breaking. Other difficulties one would encounter in
modeling oxirane photodynamics are pointed out but none of these is likely to
prevent a qualitatively correct TDDFT/TDA description of photochemistry in this
prototypical molecule.Comment: 19 pages, 17 figures, submitted to the Journal of Chemical Physic
A joint time-dependent density-functional theory for excited states of electronic systems in solution
We present a novel joint time-dependent density-functional theory for the
description of solute-solvent systems in time-dependent external potentials.
Starting with the exact quantum-mechanical action functional for both electrons
and nuclei, we systematically eliminate solvent degrees of freedom and thus
arrive at coarse-grained action functionals which retain the highly accurate
\emph{ab initio} description for the solute and are, in principle, exact. This
procedure allows us to examine approximations underlying popular embedding
theories for excited states. Finally, we introduce a novel approximate action
functional for the solute-water system and compute the solvato-chromic shift of
the lowest singlet excited state of formaldehyde in aqueous solution, which is
in good agreement with experimental findings.Comment: 11 page
A new and efficient approach to time-dependent density-functional perturbation theory for optical spectroscopy
Using a super-operator formulation of linearized time-dependent
density-functional theory, the dynamical polarizability of a system of
interacting electrons is given a matrix continued-fraction representation whose
coefficients can be obtained from the non-symmetric block-Lanczos method. The
resulting algorithm allows for the calculation of the {\em full spectrum} of a
system with a computational workload which is only a few times larger than that
needed for {\em static} polarizabilities within time-independent
density-functional perturbation theory. The method is demonstrated with the
calculation of the spectrum of benzene, and prospects for its application to
the large-scale calculation of optical spectra are discussed.Comment: 4 pages, 2 figure
Rydberg transition frequencies from the Local Density Approximation
A method is given that extracts accurate Rydberg excitations from LDA density
functional calculations, despite the short-ranged potential. For the case of He
and Ne, the asymptotic quantum defects predicted by LDA are in less than 5%
error, yielding transition frequency errors of less than 0.1eV.Comment: 4 pages, 6 figures, submitted to Phys. Rev. Let
Investigating interaction-induced chaos using time-dependent density functional theory
Systems whose underlying classical dynamics are chaotic exhibit signatures of
the chaos in their quantum mechanics. We investigate the possibility of using
time-dependent density functional theory (TDDFT) to study the case when chaos
is induced by electron-interaction alone. Nearest-neighbour level-spacing
statistics are in principle exactly and directly accessible from TDDFT. We
discuss how the TDDFT linear response procedure can reveal the mechanism of
chaos induced by electron-interaction alone. A simple model of a two-electron
quantum dot highlights the necessity to go beyond the adiabatic approximation
in TDDFT.Comment: 8 pages, 4 figure
Exchange and Correlation Kernels at the Resonance Frequency -- Implications for Excitation Energies in Density-Functional Theory
Specific matrix elements of exchange and correlation kernels in
time-dependent density-functional theory are computed. The knowledge of these
matrix elements not only constraints approximate time-dependent functionals,
but also allows to link different practical approaches to excited states,
either based on density-functional theory, or on many-body perturbation theory,
despite the approximations that have been performed to derive them.Comment: Submitted to Phys. Rev. Lett. (February 4, 1999). Other related
publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm
Optical excitations in organic molecules, clusters and defects studied by first-principles Green's function methods
Spectroscopic and optical properties of nanosystems and point defects are
discussed within the framework of Green's function methods. We use an approach
based on evaluating the self-energy in the so-called GW approximation and
solving the Bethe-Salpeter equation in the space of single-particle
transitions. Plasmon-pole models or numerical energy integration, which have
been used in most of the previous GW calculations, are not used. Fourier
transforms of the dielectric function are also avoided. This approach is
applied to benzene, naphthalene, passivated silicon clusters (containing more
than one hundred atoms), and the F center in LiCl. In the latter, excitonic
effects and the defect line are identified in the energy-resolved
dielectric function. We also compare optical spectra obtained by solving the
Bethe-Salpeter equation and by using time-dependent density functional theory
in the local, adiabatic approximation. From this comparison, we conclude that
both methods give similar predictions for optical excitations in benzene and
naphthalene, but they differ in the spectra of small silicon clusters. As
cluster size increases, both methods predict very low cross section for
photoabsorption in the optical and near ultra-violet ranges. For the larger
clusters, the computed cross section shows a slow increase as function of
photon frequency. Ionization potentials and electron affinities of molecules
and clusters are also calculated.Comment: 9 figures, 5 tables, to appear in Phys. Rev. B, 200
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