611 research outputs found
Phonons and related properties of extended systems from density-functional perturbation theory
This article reviews the current status of lattice-dynamical calculations in
crystals, using density-functional perturbation theory, with emphasis on the
plane-wave pseudo-potential method. Several specialized topics are treated,
including the implementation for metals, the calculation of the response to
macroscopic electric fields and their relevance to long wave-length vibrations
in polar materials, the response to strain deformations, and higher-order
responses. The success of this methodology is demonstrated with a number of
applications existing in the literature.Comment: 52 pages, 14 figures, submitted to Review of Modern Physic
Software for the frontiers of quantum chemistry : An overview of developments in the Q-Chem 5 package
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design.This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.Peer reviewe
Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design
Progress in Time-Dependent Density-Functional Theory
The classic density-functional theory (DFT) formalism introduced by
Hohenberg, Kohn, and Sham in the mid-1960s, is based upon the idea that the
complicated N-electron wavefunction can be replaced with the mathematically
simpler 1-electron charge density in electronic struc- ture calculations of the
ground stationary state. As such, ordinary DFT is neither able to treat
time-dependent (TD) problems nor describe excited electronic states. In 1984,
Runge and Gross proved a theorem making TD-DFT formally exact. Information
about electronic excited states may be obtained from this theory through the
linear response (LR) theory formalism. Begin- ning in the mid-1990s, LR-TD-DFT
became increasingly popular for calculating absorption and other spectra of
medium- and large-sized molecules. Its ease of use and relatively good accuracy
has now brought LR-TD-DFT to the forefront for this type of application. As the
number and the diversity of applications of TD-DFT has grown, so too has grown
our understanding of the strengths and weaknesses of the approximate
functionals commonly used for TD-DFT. The objective of this article is to
continue where a previous review of TD-DFT in this series [Annu. Rev. Phys.
Chem. 55: 427 (2004)] left off and highlight some of the problems and solutions
from the point of view of applied physical chemistry. Since doubly-excited
states have a particularly important role to play in bond dissociation and
formation in both thermal and photochemistry, particular emphasis will be
placed upon the problem of going beyond or around the TD-DFT adiabatic
approximation which limits TD-DFT calculations to nominally singly-excited
states. Posted with permission from the Annual Review of Physical Chemistry,
Volume 63 \c{opyright} 2012 by Annual Reviews, http://www.annualreviews.org
Software for the frontiers of quantum chemistry:An overview of developments in the Q-Chem 5 package
This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design
Variational Principle of Bogoliubov and Generalized Mean Fields in Many-Particle Interacting Systems
The approach to the theory of many-particle interacting systems from a
unified standpoint, based on the variational principle for free energy is
reviewed. A systematic discussion is given of the approximate free energies of
complex statistical systems. The analysis is centered around the variational
principle of N. N. Bogoliubov for free energy in the context of its
applications to various problems of statistical mechanics and condensed matter
physics. The review presents a terse discussion of selected works carried out
over the past few decades on the theory of many-particle interacting systems in
terms of the variational inequalities. It is the purpose of this paper to
discuss some of the general principles which form the mathematical background
to this approach, and to establish a connection of the variational technique
with other methods, such as the method of the mean (or self-consistent) field
in the many-body problem, in which the effect of all the other particles on any
given particle is approximated by a single averaged effect, thus reducing a
many-body problem to a single-body problem. The method is illustrated by
applying it to various systems of many-particle interacting systems, such as
Ising and Heisenberg models, superconducting and superfluid systems, strongly
correlated systems, etc. It seems likely that these technical advances in the
many-body problem will be useful in suggesting new methods for treating and
understanding many-particle interacting systems. This work proposes a new,
general and pedagogical presentation, intended both for those who are
interested in basic aspects, and for those who are interested in concrete
applications.Comment: 60 pages, Refs.25
Modelling electrooxidation of glycerol and methanol on close-packed transition metal surfaces
Burning fossil fuels leads to excess CO2 in the atmosphere, causing global warming, threatening civilisation and ecosystems worldwide. As a step in making the society fossil-independent, we need to replace oil, coal, and gas in the transportation sector with fuels originating from sustainable energy sources. Biodiesel is one such option, from which glycerol is a byproduct. With the help of electrooxidation, glycerol can be used as a feedstock to extract hydrogen gas, which may be used for upgrading biofuels or in proton exchange membrane (PEM) fuel cells. Methanol is a possible fuel in direct methanol fuel cells (DMFCs) and can, moreover, be used as a simple model for glycerol in some respects.The primary focus of this thesis is to study the reaction thermodynamics of glycerol electrooxidation on Au(111) and other close-packed late transition metal surfaces. This provides routes and products that are thermodynamically favourable, information on steps that are difficult to overcome, and at what theoretical limiting potential the reaction becomes spontaneous. Using scaling relations for adsorption energies, these results can be generalised to alloys and other possible electrode materials. We use density functional theory to model the system, and to some extent experimental verification by cyclic voltammetry. Long range dispersion (van der Waals), which have been neglected in computations until recently, is investigated by assessing density van der Waals functionals. This is of particular importance for an inert metal such as gold. Another aspect that has commonly been ignored is solvent effects, which we study for the model system of methanol electrooxidation on Au(111). This includes an implicit model - a continuous dielectric -and an explicit model of water molecules
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