174 research outputs found
Ab initio calculations of response properties including electron-hole interaction
We discuss the current status of a computational approach which allows to
evaluate the dielectric matrix, and hence electronic excitations like optical
properties, including local field and excitonic effects. We introduce a recent
numerical development which greatly reduces the use of memory in such type of
calculations, and hence eliminates one of the bottlenecks for the application
to complex systems. We present recent applications of the method, focusing our
interest on insulating oxides.Comment: 11 pages, 5 figures, 1999 MRS Proceedin
First-principles GW calculations for fullerenes, porphyrins, phtalocyanine, and other molecules of interest for organic photovoltaic applications
We evaluate the performances of ab initio GW calculations for the ionization
energies and HOMO-LUMO gaps of thirteen gas phase molecules of interest for
organic electronic and photovoltaic applications, including the C60 fullerene,
pentacene, free-base porphyrins and phtalocyanine, PTCDA, and standard monomers
such as thiophene, fluorene, benzothiazole or thiadiazole. Standard G0W0
calculations, that is starting from eigenstates obtained with local or
semilocal functionals, significantly improve the ionization energy and band gap
as compared to density functional theory Kohn-Sham results, but the calculated
quasiparticle values remain too small as a result of overscreening. Starting
from Hartree-Fock-like eigenvalues provides much better results and is
equivalent to performing self-consistency on the eigenvalues, with a resulting
accuracy of 2~4% as compared to experiment. Our calculations are based on an
efficient gaussian-basis implementation of GW with explicit treatment of the
dynamical screening through contour deformation techniques.Comment: 10 pages, 3 figure
Can molecular projected density-of-states (PDOS) be systematically used in electronic conductance analysis?
Using benzene-diamine and benzene-dithiol molecular junctions as benchmarks,
we investigate the widespread analysis of the quantum transport conductance
in terms of the projected density of states (PDOS) onto
molecular orbitals (MOs). We first consider two different methods for
identifying the relevant MOs: 1) diagonalization of the Hamiltonian of the
isolated molecule, and 2) diagonalization of a submatrix of the junction
Hamiltonian constructed by considering only basis elements localized on the
molecule. We find that these two methods can lead to substantially different
MOs and hence PDOS. Furthermore, within Method 1, the PDOS can differ depending
on the isolated molecule chosen to represent the molecular junction (e.g.
benzene-dithiol or -dithiolate); and, within Method 2, the PDOS depends on the
chosen basis set. We show that these differences can be critical when the PDOS
is used to provide a physical interpretation of the conductance (especially,
when it has small values as it happens typically at zero bias). In this work,
we propose a new approach trying to reconcile the two traditional methods.
Though some improvements are achieved, the main problems are still unsolved.
Our results raise more general questions and doubts on a PDOS-based analysis of
the conductance.Comment: 12 pages, 9 figure
Ground-state correlation energy of beryllium dimer by the Bethe-Salpeter equation
Since the '30s the interatomic potential of the beryllium dimer Be has
been both an experimental and a theoretical challenge. Calculating the
ground-state correlation energy of Be along its dissociation path is a
difficult problem for theory. We present ab initio many-body perturbation
theory calculations of the Be interatomic potential using the GW
approximation and the Bethe-Salpeter equation (BSE). The ground-state
correlation energy is calculated by the trace formula with checks against the
adiabatic-connection fluctuation-dissipation theorem formula. We show that
inclusion of GW corrections already improves the energy even at the level of
the random-phase approximation. At the level of the BSE on top of the GW
approximation, our calculation is in surprising agreement with the most
accurate theories and with experiment. It even reproduces an experimentally
observed flattening of the interatomic potential due to a delicate correlations
balance from a competition between covalent and van der Waals bonding.Comment: 6 pages, 2 figures, 1 tabl
Beyond time-dependent exact-exchange: the need for long-range correlation
In the description of the interaction between electrons beyond the classical
Hartree picture, bare exchange often yields a leading contribution. Here we
discuss its effect on optical spectra of solids, comparing three different
frameworks: time-dependent Hartree-Fock, a recently introduced combined
density-functional and Green's functions approach applied to the bare exchange
self-energy, and time-dependent exact-exchange within time-dependent
density-functional theory (TD-EXX). We show that these three approximations
give rise to identical excitonic effects in solids; these effects are
drastically overestimated for semiconductors. They are partially compensated by
the usual overestimation of the quasiparticle band gap within Hartree-Fock. The
physics that lacks in these approaches can be formulated as screening. We show
that the introduction of screening in TD-EXX indeed leads to a formulation that
is equivalent to previously proposed functionals derived from Many-Body
Perturbation Theory. It can be simulated by reducing the long-range part of the
Coulomb interaction: this produces absorption spectra of semiconductors in good
agreement with experiment.Comment: 12 pages, 3 figures, 1 tabl
Many-body correlations and coupling in benzene-dithiol junctions
Most theoretical studies of nanoscale transport in molecular junctions rely
on the combination of the Landauer formalism with Kohn-Sham density functional
theory (DFT) using standard local and semilocal functionals to approximate
exchange and correlation effects. In many cases, the resulting conductance is
overestimated with respect to experiments. Recent works have demonstrated that
this discrepancy may be reduced when including many-body corrections on top of
DFT. Here we study benzene-dithiol (BDT) gold junctions and analyze the effect
of many-body perturbation theory (MBPT) on the calculation of the conductance
with respect to different bonding geometries. We find that the many-body
corrections to the conductance strongly depend on the metal-molecule coupling
strength. In the BDT junction with the lowest coupling, many-body corrections
reduce the overestimation on the conductance to a factor two, improving the
agreement with experiments. In contrast, in the strongest coupling cases,
many-body corrections on the conductance are found to be sensibly smaller and
standard DFT reveals a valid approach.Comment: 9 pages, 4 figure
Optical absorption in small BN and C nanotubes
We present a theoretical study of the optical absorption spectrum of small
boron-nitride and carbon nanotubes using time-dependent density-functional
theory and the random phase approximation. Both for C and BN tubes, the
absorption of light polarized perpendicular to the tube-axis is strongly
suppressed due to local field effects. Since BN-tubes are wide band-gap
insulators, they only absorb in the ultra-violet energy regime, independently
of chirality and diameter. In comparison with the spectra of the single C and
BN-sheets, the tubes display additional fine-structure which stems from the
(quasi-) one-dimensionality of the tubes and sensitively depends on the
chirality and tube diameter. This fine structure can provide additional
information for the assignment of tube indices in high resolution optical
absorption spectroscopy.Comment: 5 pages, 3 figure
Ab initio GW many-body effects in graphene
We present an {\it ab initio} many-body GW calculation of the self-energy,
the quasiparticle band plot and the spectral functions in free-standing undoped
graphene. With respect to other approaches, we numerically take into account
the full ionic and electronic structure of real graphene and we introduce
electron-electron interaction and correlation effects from first principles.
Both non-hermitian and also dynamical components of the self-energy are fully
taken into account. With respect to DFT-LDA, the Fermi velocity is
substantially renormalized and raised by a 17%, in better agreement with
magnetotransport experiments. Furthermore, close to the Dirac point the linear
dispersion is modified by the presence of a kink, as observed in ARPES
experiments. Our calculations show that the kink is due to low-energy single-particle excitations and to the plasmon. Finally, the GW
self-energy does not open the band gap.Comment: 5 pages, 4 figures, 1 tabl
Transforming nonlocality into frequency dependence: a shortcut to spectroscopy
Measurable spectra are theoretically very often derived from complicated
many-body Green's functions. In this way, one calculates much more information
than actually needed. Here we present an in principle exact approach to
construct effective potentials and kernels for the direct calculation of
electronic spectra. In particular, the potential that yields the spectral
function needed to describe photoemission turns out to be dynamical but {\it
local} and {\it real}. As example we illustrate this ``photoemission
potential'' for sodium and aluminium, modelled as homogeneous electron gas, and
discuss in particular its frequency dependence stemming from the nonlocality of
the corresponding self-energy. We also show that our approach leads to a very
short derivation of a kernel that is known to well describe absorption and
energy-loss spectra of a wide range of materials
- …