154,376 research outputs found
First-Principles Based Matrix-Green's Function Approach to Molecular Electronic Devices: General Formalism
Transport in molecular electronic devices is different from that in
semiconductor mesoscopic devices in two important aspects: (1) the effect of
the electronic structure and (2) the effect of the interface to the external
contact. A rigorous treatment of molecular electronic devices will require the
inclusion of these effects in the context of an open system exchanging particle
and energy with the external environment. This calls for combining the theory
of quantum transport with the theory of electronic structure starting from the
first-principles. We present a rigorous yet tractable matrix Green's function
approach for studying transport in molecular electronic devices, based on the
Non-Equilibrium Green's Function Formalism of quantum transport and the
density-functional theory of electronic structure using local orbital basis
sets. By separating the device rigorously into the molecular region and the
contact region, we can take full advantage of the natural spatial locality
associated with the metallic screening in the electrodes and focus on the
physical processes in the finite molecular region. This not only opens up the
possibility of using the existing well-established technique of molecular
electronic structure theory in transport calculations with little change, but
also allows us to use the language of qualitative molecular orbital theory to
interpret and rationalize the results of the computation. For the device at
equilibrium, our method provides an alternative approach for solving the
molecular chemisorption problem. For the device out of equilibrium, we show
that the calculation of elastic current transport through molecules, both
conceptually and computationally, is no more difficult than solving the
chemisorption problem.Comment: To appear in Chemical Physic
Molecular Dynamics in a Grand Ensemble: Bergmann-Lebowitz model and Adaptive Resolution Simulation
This article deals with the molecular dynamics simulation of open systems
that can exchange energy and matter with a reservoir; the physics of the
reservoir and its interactions with the system are described by the model
introduced by Bergmann and Lebowitz.Despite its conceptual appeal, the model
did not gain popularity in the field of molecular simulation and, as a
consequence, did not play a role in the development of open system molecular
simulation techniques, even though it can provide the conceptual legitimation
of simulation techniques that mimic open systems. We shall demonstrate that the
model can serve as a tool to devise both numerical procedures and conceptual
definitions of physical quantities that cannot be defined in a straightforward
way by systems with a fixed number of molecules. In particular, we discuss the
utility of the Bergmann-Lebowitz (BL) model for the calculation of equilibrium
time correlation functions within the Grand Canonical Adaptive Resolution
method (GC-AdResS) and report numerical results for the case of liquid water.Comment: 31 pages, 6 figure
A correlated-polaron electronic propagator: open electronic dynamics beyond the Born-Oppenheimer approximation
In this work we develop a theory of correlated many-electron dynamics dressed
by the presence of a finite-temperature harmonic bath. The theory is based on
the ab-initio Hamiltonian, and thus well-defined apart from any
phenomenological choice of collective basis states or electronic coupling
model. The equation-of-motion includes some bath effects non-perturbatively,
and can be used to simulate line- shapes beyond the Markovian approximation and
open electronic dynamics which are subjects of renewed recent interest. Energy
conversion and transport depend critically on the ratio of electron-electron
coupling to bath-electron coupling, which is a fitted parameter if a
phenomenological basis of many-electron states is used to develop an electronic
equation of motion. Since the present work doesn't appeal to any such basis, it
avoids this ambiguity. The new theory produces a level of detail beyond the
adiabatic Born-Oppenheimer states, but with cost scaling like the
Born-Oppenheimer approach. While developing this model we have also applied the
time-convolutionless perturbation theory to correlated molecular excitations
for the first time. Resonant response properties are given by the formalism
without phenomenological parameters. Example propagations with a developmental
code are given demonstrating the treatment of electron-correlation in
absorption spectra, vibronic structure, and decay in an open system.Comment: 25 pages 7 figure
Optical absorption spectra of finite systems from a conserving Bethe-Salpeter equation approach
We present a method for computing optical absorption spectra by means of a
Bethe-Salpeter equation approach, which is based on a conserving linear
response calculation for electron-hole coherences in the presence of an
external electromagnetic field. This procedure allows, in principle, for the
determination of the electron-hole correlation function self-consistently with
the corresponding single-particle Green function. We analyze the general
approach for a "one-shot" calculation of the photoabsorption cross section of
finite systems, and discuss the importance of scattering and dephasing
contributions in this approach. We apply the method to the closed-shell
clusters Na_4, Na^+_9 and Na^+_(21), treating one active electron per Na atom.Comment: 9 pages, 3 figure
Long-range correlation energy calculated from coupled atomic response functions
An accurate determination of the electron correlation energy is essential for
describing the structure, stability, and function in a wide variety of systems,
ranging from gas-phase molecular assemblies to condensed matter and
organic/inorganic interfaces. Even small errors in the correlation energy can
have a large impact on the description of chemical and physical properties in
the systems of interest. In this context, the development of efficient
approaches for the accurate calculation of the long-range correlation energy
(and hence dispersion) is the main challenge. In the last years a number of
methods have been developed to augment density functional approximations via
dispersion energy corrections, but most of these approaches ignore the
intrinsic many-body nature of correlation effects, leading to inconsistent and
sometimes even qualitatively incorrect predictions. Here we build upon the
recent many-body dispersion (MBD) framework, which is intimately linked to the
random-phase approximation for the correlation energy. We separate the
correlation energy into short-range contributions that are modeled by
semi-local functionals and long-range contributions that are calculated by
mapping the complex all-electron problem onto a set of atomic response
functions coupled in the dipole approximation. We propose an effective
range-separation of the coupling between the atomic response functions that
extends the already broad applicability of the MBD method to non-metallic
materials with highly anisotropic responses, such as layered nanostructures.
Application to a variety of high-quality benchmark datasets illustrates the
accuracy and applicability of the improved MBD approach, which offers the
prospect of first-principles modeling of large structurally complex systems
with an accurate description of the long-range correlation energy.Comment: 15 pages, 3 figure
Can Density Matrix Embedding Theory with the Complete Activate Space Self-Consistent Field Solver Describe Single and Double Bond Breaking in Molecular Systems?
Density matrix embedding theory (DMET) [Phys. Rev. Lett.2012, 109, 186404]
has been demonstrated as an efficient wave-function-based embedding method to
treat extended systems. Despite its success in many quantum lattice models, the
extension of DMET to real chemical systems has been tested only on selected
cases. Herein, we introduce the use of the complete active space
self-consistent field (CASSCF) method as a correlated impurity solver for DMET,
leading to a method called CAS-DMET. We test its performance in describing the
dissociation of a H-H single bond in a H10 ring model system and an N=N double
bond in azomethane (CH3-N=N-CH3) and pentyldiazene (CH3(CH2)4-N=NH). We find
that the performance of CAS-DMET is comparable to CASSCF with different active
space choices when single-embedding DMET corresponding to only one embedding
problem for the system is used. When multiple embedding problems are used for
the system, the CAS-DMET is in a good agreement with CASSCF for the geometries
around the equilibrium, but not in equal agreement at bond dissociation.Comment: 28 pages, 9 figures, TOC graphi
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