115 research outputs found
Resonant hot charge-transfer excitations in fullerene-porphyrin complexes: a many-body Bethe-Salpeter study
We study within the many-body Green's function GW and Bethe-Salpeter
approaches the neutral singlet excitations of the zinctetraphenylporphyrin and
C70 fullerene donor-acceptor complex. The lowest transition is a
charge-transfer excitation between the donor and the acceptor with an energy in
excellent agreement with recent constrained density functional theory
calculations. Beyond the lowest charge-transfer state, of which the energy can
be determined with simple electrostatic models that we validate, the
Bethe-Salpeter approach provides the full excitation spectrum. We evidence the
existence of hot electron-hole states which are resonant in energy with the
lowest donor intramolecular excitation and show an hybrid intramolecular and
charge-transfer character, favouring the transition towards charge separation.
These findings support the recently proposed scenario for charge separation at
donor-acceptor interfaces through delocalized hot charge-transfer states.Comment: 9 pages, 4 figure
Atomistic calculation of the thermal conductance of large scale bulk-nanowire junctions
We have developed an efficient scalable kernel method for thermal transport
in open systems, with which we have computed the thermal conductance of a
junction between bulk silicon and silicon nanowires with diameter up to 10 nm.
We have devised scaling laws for transmission and reflection spectra, which
allow us to predict the thermal resistance of bulk-nanowire interfaces with
larger cross sections than those achievable with atomistic simulations. Our
results indicate the characteristic size beyond which atomistic systems can be
treated accurately by mesoscopic theories.Comment: 6 pages, 4 figure
Atomistic simulations of heat transport in real-scale silicon nanowire devices
Utilizing atomistic lattice dynamics and scattering theory, we study thermal
transport in nanodevices made of 10 nm thick silicon nanowires, from 10 to 100
nm long, sandwiched between two bulk reservoirs. We find that thermal transport
in devices differs significantly from that of suspended extended nanowires, due
to phonon scattering at the contact interfaces. We show that thermal
conductance and the phonon transport regime can be tuned from ballistic to
diffusive by varying the surface roughness of the nanowires and their length.
In devices containing short crystalline wires phonon tunneling occurs and
enhances the conductance beyond that of single contacts.Comment: 5 pages, 5 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
Ab initio investigation of the melting line of nitrogen at high pressure
Understanding the behavior of molecular systems under pressure is a
fundamental problem in condensed matter physics. In the case of nitrogen, the
determination of the phase diagram and in particular of the melting line, are
largely open problems. Two independent experiments have reported the presence
of a maximum in the nitrogen melting curve, below 90 GPa, however the position
and the interpretation of the origin of such maximum differ. By means of ab
initio molecular dynamics simulations based on density functional theory and
thermodynamic integration techniques, we have determined the phase diagram of
nitrogen in the range between 20 and 100 GPa. We find a maximum in the melting
line, related to a transformation in the liquid, from molecular N_2 to
polymeric nitrogen accompanied by an insulator-to-metal transition
Many-body Green's function GW and Bethe-Salpeter study of the optical excitations in a paradigmatic model dipeptide
We study within the many-body Green's function GW and Bethe-Salpeter
formalisms the excitation energies of a paradigmatic model dipeptide, focusing
on the four lowest-lying local and charge-transfer excitations. Our GW
calculations are performed at the self-consistent level, updating first the
quasiparticle energies, and further the single-particle wavefunctions within
the static Coulomb-hole plus screened-exchange approximation to the GW
self-energy operator. Important level crossings, as compared to the starting
Kohn-Sham LDA spectrum, are identified. Our final Bethe-Salpeter singlet
excitation energies are found to agree, within 0.07 eV, with CASPT2 reference
data, except for one charge-transfer state where the discrepancy can be as
large as 0.5 eV. Our results agree best with LC-BLYP and CAM-B3LYP calculations
with enhanced long-range exchange, with a 0.1 eV mean absolute error. This has
been achieved employing a parameter-free formalism applicable to metallic or
insulating extended or finite systems.Comment: 25 pages, 5 figure
Accurate Complex Scaling of Three Dimensional Numerical Potentials
The complex scaling method, which consists in continuing spatial coordinates
into the complex plane, is a well-established method that allows to compute
resonant eigenfunctions of the time-independent Schroedinger operator. Whenever
it is desirable to apply the complex scaling to investigate resonances in
physical systems defined on numerical discrete grids, the most direct approach
relies on the application of a similarity transformation to the original,
unscaled Hamiltonian. We show that such an approach can be conveniently
implemented in the Daubechies wavelet basis set, featuring a very promising
level of generality, high accuracy, and no need for artificial convergence
parameters. Complex scaling of three dimensional numerical potentials can be
efficiently and accurately performed. By carrying out an illustrative resonant
state computation in the case of a one-dimensional model potential, we then
show that our wavelet-based approach may disclose new exciting opportunities in
the field of computational non-Hermitian quantum mechanics.Comment: 11 pages, 8 figure
Mechanical Tuning of Thermal Transport in a Molecular Junction
Understanding and controlling heat transport in molecular junctions would
provide new routes to design nanoscale coupled electronic and phononic devices.
Using first principles full quantum calculations, we tune thermal conductance
of a molecular junction by mechanically compressing and extending a short
alkane chain connected to graphene leads. We find that the thermal conductance
of the compressed junction drops by half in comparison to the extended
junction, making it possible to turn on and off the heat current. The low
conductance of the off state does not vary by further approaching the leads and
stems from the suppression of the transmission of the in--plane transverse and
longitudinal channels. Furthermore, we show that misalignment of the leads does
not reduce the conductance ratio. These results also contribute to the general
understanding of thermal transport in molecular junctions.Comment: 12 pages, 6 figure
Many-body calculations with very large scale polarizable environments made affordable: a fully ab initio QM/QM approach
We present a many-body formalism for quantum subsystems embedded in
discrete polarizable environments containing up to several hundred thousand
atoms described at a fully ab initio random phase approximation level. Our
approach is based on a fragment approximation in the construction of the
Green's function and independent-electron susceptibilities. Further, the
environing fragments susceptibility matrices are reduced to a minimal but
accurate representation preserving low order polarizability tensors through a
constrained minimization scheme. This approach dramatically reduces the cost
associated with inverting the Dyson equation for the screened Coulomb potential
, while preserving the description of short to long-range screening effects.
The efficiency and accuracy of the present scheme is exemplified in the
paradigmatic cases of fullerene bulk, surface, subsurface, and slabs with
varying number of layers
Static versus dynamically polarizable environments within the many-body formalism
Continuum or discrete polarizable models for the study of optoelectronic
processes in embedded subsystems rely mostly on the restriction of the
surrounding electronic dielectric response to its low frequency limit. Such a
description hinges on the assumption that the electrons in the surrounding
medium react instantaneously to any excitation in the central subsystem,
treating thus the environment in the adiabatic limit. Exploiting a recently
developed embedded formalism, with an environment described at the fully
ab initio level, we assess the merits of the adiabatic limit with respect to an
environment where the full dynamics of the dielectric response is considered.
Further, we show how to properly take the static limit of the environment
susceptibility, introducing the so-called Coulomb-hole and screened-exchange
contributions to the reaction field. As a first application, we consider a
C molecule at the surface of a C crystal, namely a case where the
dynamics of the embedded and embedding subsystems are similar. The common
adiabatic assumption, when properly treated, generates errors below on
the polarization energy associated with frontier energy levels and associated
energy gaps. Finally, we consider a water molecule inside a metallic nanotube,
the worst case for the environment adiabatic limit. The error on the gap
polarization energy remains below , even though the error on the frontier
orbitals polarization energies can reach a few tenths of an electronvolt
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