3,663 research outputs found
Computational characterization and prediction of metal-organic framework properties
In this introductory review, we give an overview of the computational
chemistry methods commonly used in the field of metal-organic frameworks
(MOFs), to describe or predict the structures themselves and characterize their
various properties, either at the quantum chemical level or through classical
molecular simulation. We discuss the methods for the prediction of crystal
structures, geometrical properties and large-scale screening of hypothetical
MOFs, as well as their thermal and mechanical properties. A separate section
deals with the simulation of adsorption of fluids and fluid mixtures in MOFs
ATK-ForceField: A New Generation Molecular Dynamics Software Package
ATK-ForceField is a software package for atomistic simulations using
classical interatomic potentials. It is implemented as a part of the Atomistix
ToolKit (ATK), which is a Python programming environment that makes it easy to
create and analyze both standard and highly customized simulations. This paper
will focus on the atomic interaction potentials, molecular dynamics, and
geometry optimization features of the software, however, many more advanced
modeling features are available. The implementation details of these algorithms
and their computational performance will be shown. We present three
illustrative examples of the types of calculations that are possible with
ATK-ForceField: modeling thermal transport properties in a silicon germanium
crystal, vapor deposition of selenium molecules on a selenium surface, and a
simulation of creep in a copper polycrystal.Comment: 28 pages, 9 figure
On the inner workings of Monte Carlo codes
We review state-of-the-art Monte Carlo (MC) techniques for computing fluid coexistence properties (Gibbs simulations) and adsorption simulations in nanoporous materials such as zeolites and metal-organic frameworks. Conventional MC is discussed and compared to advanced techniques such as reactive MC, configurational-bias Monte Carlo and continuous fractional MC. The latter technique overcomes the problem of low insertion probabilities in open systems. Other modern methods are (hyper-)parallel tempering, Wang-Landau sampling and nested sampling. Details on the techniques and acceptance rules as well as to what systems these techniques can be applied are provided. We highlight consistency tests to help validate and debug MC codes
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Quantum corrected full-band semiclassical Monte Carlo simulation research of charge transport in Si, stressed-Si, and SiGe MOSFETs
This Ph.D. research is centered around a full-band Monte Carlo device simulator
(“Monte Carlo at the University of Texas”, MCUT) with quantum corrections
(based on one-dimensional Schrödinger equation solver). The code itself was
based on a solid infrastructure of a Monte Carlo simulator, “MoCa” from the
University of Illinois at Urbana-Champaign. To that there were added new
methods and features during my Ph.D. program, including strained band
structures, alternative (to conventional 100 ) surface orientations, full-band
scattering mechanisms, and valley-dependent quantum correction. These
features enable “MCUT” to be used to model various strained and/or alloyed
silicon MOSFETs, as well as the MOSFETs composed of alternative materials
such as Ge, in sub-100 nm regime. Monte Carlo simulation, itself, handles short
channel effects and hot carriers in ultra small device well; full-band structure
replaces the inaccurate and unknown (for new/strained materials) analytical
formulae; and the quantum corrections approximate quantum-confinement effects
on device performance. The goal is to understand and predict the device
behavior of the so called “non-classical” CMOS ― beyond bulk Si based
CMOS ― in the sub-100 nm regime.Electrical and Computer Engineerin
Modeling techniques for quantum cascade lasers
Quantum cascade lasers are unipolar semiconductor lasers covering a wide
range of the infrared and terahertz spectrum. Lasing action is achieved by
using optical intersubband transitions between quantized states in specifically
designed multiple-quantum-well heterostructures. A systematic improvement of
quantum cascade lasers with respect to operating temperature, efficiency and
spectral range requires detailed modeling of the underlying physical processes
in these structures. Moreover, the quantum cascade laser constitutes a
versatile model device for the development and improvement of simulation
techniques in nano- and optoelectronics. This review provides a comprehensive
survey and discussion of the modeling techniques used for the simulation of
quantum cascade lasers. The main focus is on the modeling of carrier transport
in the nanostructured gain medium, while the simulation of the optical cavity
is covered at a more basic level. Specifically, the transfer matrix and finite
difference methods for solving the one-dimensional Schr\"odinger equation and
Schr\"odinger-Poisson system are discussed, providing the quantized states in
the multiple-quantum-well active region. The modeling of the optical cavity is
covered with a focus on basic waveguide resonator structures. Furthermore,
various carrier transport simulation methods are discussed, ranging from basic
empirical approaches to advanced self-consistent techniques. The methods
include empirical rate equation and related Maxwell-Bloch equation approaches,
self-consistent rate equation and ensemble Monte Carlo methods, as well as
quantum transport approaches, in particular the density matrix and
non-equilibrium Green's function (NEGF) formalism. The derived scattering rates
and self-energies are generally valid for n-type devices based on
one-dimensional quantum confinement, such as quantum well structures
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