2,131 research outputs found
Benchmarking GW against exact diagonalization for semi-empirical models
We calculate groundstate total energies and single-particle excitation
energies of seven pi conjugated molecules described with the semi-empirical
Pariser-Parr-Pople (PPP) model using self-consistent many-body perturbation
theory at the GW level and exact diagonalization. For the total energies GW
captures around 65% of the groundstate correlation energy. The lowest lying
excitations are overscreened by GW leading to an underestimation of electron
affinities and ionization potentials by approximately 0.15 eV corresponding to
2.5%. One-shot G_0W_0 calculations starting from Hartree-Fock reduce the
screening and improve the low-lying excitation energies. The effect of the GW
self-energy on the molecular excitation energies is shown to be similar to the
inclusion of final state relaxations in Hartree-Fock theory. We discuss the
break down of the GW approximation in systems with short range interactions
(Hubbard models) where correlation effects dominate over screening/relaxation
effects. Finally we illustrate the important role of the derivative
discontinuity of the true exchange-correlation functional by computing the
exact Kohn-Sham levels of benzene.Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev.
First-principles GW-BSE excitations in organic molecules
We present a first-principles method for the calculation of optical
excitations in nanosystems. The method is based on solving the Bethe-Salpeter
equation (BSE) for neutral excitations. The electron self-energy is evaluated
within the GW approximation, with dynamical screening effects described within
time-dependent density-functional theory in the adiabatic, local approximation.
This method is applied to two systems: the benzene molecule, CH, and
azobenzene, CHN. We give a description of the
photoisomerization process of azobenzene after an excitation,
which is consistent with multi-configuration calculations
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
Kohn-Sham decomposition in real-time time-dependent density-functional theory: An efficient tool for analyzing plasmonic excitations
The real-time-propagation formulation of time-dependent density-functional
theory (RT-TDDFT) is an efficient method for modeling the optical response of
molecules and nanoparticles. Compared to the widely adopted linear-response
TDDFT approaches based on, e.g., the Casida equations, RT-TDDFT appears,
however, lacking efficient analysis methods. This applies in particular to a
decomposition of the response in the basis of the underlying single-electron
states. In this work, we overcome this limitation by developing an analysis
method for obtaining the Kohn-Sham electron-hole decomposition in RT-TDDFT. We
demonstrate the equivalence between the developed method and the Casida
approach by a benchmark on small benzene derivatives. Then, we use the method
for analyzing the plasmonic response of icosahedral silver nanoparticles up to
Ag. Based on the analysis, we conclude that in small nanoparticles
individual single-electron transitions can split the plasmon into multiple
resonances due to strong single-electron-plasmon coupling whereas in larger
nanoparticles a distinct plasmon resonance is formed.Comment: 11 pages, 3 figure
Performance of a non-empirical meta-GGA density functional for excitation energies
It is known that the adiabatic approximation in time-dependent density
functional theory usually provides a good description of low-lying excitations
of molecules. In the present work, the capability of the adiabatic nonempirical
meta-generalized gradient approximation (meta-GGA) of Tao, Perdew, Staroverov,
and Scuseria (TPSS) to describe atomic and molecular excitations is tested. The
adiabatic (one-parameter) hybrid version of the TPSS meta-GGA and the adiabatic
GGA of Perdew, Burke, and Ernzerhof (PBE) are also included in the test. The
results are compared to experiments and to two well-established hybrid
functionals PBE0 and B3LYP. Calculations show that both adiabatic TPSS and
TPSSh functionals produce excitation energies in fairly good agreement with
experiments, and improve upon the adiabatic local spin density approximation
and, in particular, the adiabatic PBE GGA. This further confirms that TPSS is
indeed a reliable nonhybrid universal functional which can serve as the
starting point from which higher-level approximations can be constructed. The
systematic underestimate of the low-lying vertical excitation energies of
molecules with time-dependent density functionals within the adiabatic
approximation suggests that further improvement can be made with nonadiabatic
corrections.Comment: 7 page
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