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H. Appel

Bio: H. Appel is an academic researcher from Max Planck Society. The author has contributed to research in topics: Time-dependent density functional theory & Full configuration interaction. The author has an hindex of 8, co-authored 10 publications receiving 1019 citations. Previous affiliations of H. Appel include Rutgers University & Free University of Berlin.

Papers
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Journal ArticleDOI
TL;DR: The octopus project as mentioned in this paper is a large-scale parallelization of density-functional theory in the ground state and time-dependent density functional theory for dynamical effects, with a focus on the optical (i.e. electronic) linear response properties of nanostructures and biomolecules.
Abstract: We report on the background, current status, and current lines of development of the octopus project. This program materializes the main equations of density-functional theory in the ground state, and of time-dependent density-functional theory for dynamical effects. The focus is nowadays placed on the optical (i.e. electronic) linear response properties of nanostructures and biomolecules, and on the non-linear response to high-intensity fields of finite systems, with particular attention to the coupled ionic-electronic motion (i.e. photo-chemical processes). In addition, we are currently extending the code to the treatment of periodic systems (both to one-dimensional chains, two-dimensional slabs, or fully periodic solids), magnetic properties (ground state properties and excitations), and to the field of quantum-mechanical transport or “molecular electronics.” In this communication, we concentrate on the development of the methodology: we review the essential numerical schemes used in the code, and report on the most recent implementations, with special attention to the introduction of adaptive coordinates, to the extension of our real-space technique to tackle periodic systems, and on large-scale parallelization. More information on the code, as well as the code itself, can be found at http://www.tddft.org/programs/octopus/. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

788 citations

Journal ArticleDOI
TL;DR: An approximate solution to the time-dependent density-functional theory response equations for finite systems is developed, yielding corrections to the single-pole approximation, which explains why allowed Kohn-Sham transition frequencies and oscillator strengths are usually good approximations to the true values.
Abstract: An approximate solution to the time-dependent density-functional theory response equations for finite systems is developed, yielding corrections to the single-pole approximation. These explain why allowed Kohn-Sham transition frequencies and oscillator strengths are usually good approximations to the true values, and why sometimes they are not. The approximation yields simple expressions for Gorling-Levy perturbation theory results, and a method for estimating expectation values of the unknown exchange-correlation kernel. DOI: 10.1103/PhysRevLett.90.043005 cost more, limiting their use to smaller systems. TDDFT has recently been applied to electron-transfer problems in biological systems (7). Although TDDFT methodology has been implemented and is being used widely, understanding of its accuracy and reliability, as well as its relation to other methods, has been slow. The relation to first-order Gorling-Levy (GL) perturbation theory has been found (8,9), as well as the connection with the GW approximation (8). The extreme case of stretched H2 has recently been studied by several authors (10 -12), although this also represents difficulties for the ground-state theory (13). By using a matrix for- mulation of Casida (14), the present paper shows how, when the excitations of a system are discrete, an approxi- mate solution, that can be made arbitrarily accurate, can be used to understand and explain many trends in the results of TDDFT calculations.

159 citations

Journal ArticleDOI
02 Jun 2020
TL;DR: In this article, a coupled-cluster theory for systems of electrons strongly coupled to photons was developed, providing a promising theoretical tool in polaritonic chemistry with a perspective of application to all types of fermion-boson coupled systems.
Abstract: We develop coupled-cluster theory for systems of electrons strongly coupled to photons, providing a promising theoretical tool in polaritonic chemistry with a perspective of application to all types of fermion-boson coupled systems. We show benchmark results for model molecular Hamiltonians coupled to cavity photons. By comparing to full configuration interaction results for various ground-state properties and optical spectra, we demonstrate that our method captures all key features present in the exact reference, including Rabi splittings and multiphoton processes. Furthermore, a path on how to incorporate our bosonic extension of coupled-cluster theory into existing quantum chemistry programs is given.

47 citations

Journal ArticleDOI
01 Oct 2010-EPL
TL;DR: In this paper, it was shown that adiabatic extensions of common functionals employed in ground-state reduced-density-matrix functional theory have the shortcoming of leading to occupation numbers independent of time.
Abstract: Using the equations of motion for the occupation numbers of natural spin orbitals we show that adiabatic extensions of common functionals employed in ground-state reduced-density-matrix-functional theory have the shortcoming of leading to occupation numbers independent of time. We illustrate the exact time-dependence of the natural spin orbitals and occupation numbers for two strongly nonlinear cases, namely electron-ion scattering and atoms in strong laser fields. In the latter case, we observe strong variations of the occupation numbers in time.

39 citations

Journal ArticleDOI
TL;DR: In this article, the occupation number of the natural spin orbitals of the electron-ion scattering and for atoms in strong laser fields has been investigated and shown to be independent of time.
Abstract: We report equations of motion for the occupation numbers of natural spin orbitals and show that adiabatic extensions of common functionals employed in ground-state reduced-density-matrix-functional theory have the shortcoming of leading always to occupation numbers which are independent of time. We illustrate the exact time-dependence of the natural spin orbitals and occupation numbers for the case of electron-ion scattering and for atoms in strong laser fields. In the latter case, we observe strong variations of the occupation numbers in time.

31 citations


Cited by
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TL;DR: It was found that LC-TDDFT clearly reproduces the correct asymptotic behavior of the charge-transfer excitation energy of ethylene-tetrafluoroethylene dimer for the long intramolecular distance, unlike a conventional far-nucleus asymPTotic correction scheme.
Abstract: We apply the long-range correction (LC) scheme for exchange functionals of density functional theory to time-dependent density functional theory (TDDFT) and examine its efficiency in dealing with the serious problems of TDDFT, i.e., the underestimations of Rydberg excitation energies, oscillator strengths, and charge-transfer excitation energies. By calculating vertical excitation energies of typical molecules, it was found that LC-TDDFT gives accurate excitation energies, within an error of 0.5 eV, and reasonable oscillator strengths, while TDDFT employing a pure functional provides 1.5 eV lower excitation energies and two orders of magnitude lower oscillator strengths for the Rydberg excitations. It was also found that LC-TDDFT clearly reproduces the correct asymptotic behavior of the charge-transfer excitation energy of ethylene-tetrafluoroethylene dimer for the long intramolecular distance, unlike a conventional far-nucleus asymptotic correction scheme. It is, therefore, presumed that poor TDDFT results for pure functionals may be due to their lack of a long-range orbital-orbital interaction.

1,668 citations

Journal ArticleDOI
TL;DR: In this paper, the electrostatic attraction between the separated charges in long-range excited charge-transfer states originates from the non-local Hartree-Fock exchange potential and is a nonlocal property.
Abstract: The electrostatic attraction between the separated charges in long-range excited charge-transfer states originates from the non-local Hartree-Fock exchange potential and is, thus, a non-local property. Present-day time-dependent density functional theory employing local exchange-correlation functionals does not capture this effect and therefore fails to describe charge-transfer excited states correctly. A hybrid method that is qualitatively correct is described.

1,455 citations

Journal ArticleDOI
TL;DR: In this article, the authors introduce density functional theory and review recent progress in its application to transition metal chemistry, including local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and solids.
Abstract: We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

1,449 citations

Journal ArticleDOI
TL;DR: An overview of TDDFT from its theoretical foundations to several applications both in the linear and in the nonlinear regime is given.
Abstract: Time-dependent density functional theory (TDDFT) can be viewed as an exact reformulation of time-dependent quantum mechanics, where the fundamental variable is no longer the many-body wave function but the density. This time-dependent density is determined by solving an auxiliary set of noninteracting Schrodinger equations, the Kohn-Sham equations. The nontrivial part of the many-body interaction is contained in the so-called exchange-correlation potential, for which reasonably good approximations exist. Within TDDFT two regimes can be distinguished: (a) If the external time-dependent potential is "small," the complete numerical solution of the time-dependent Kohn-Sham equations can be avoided by the use of linear response theory. This is the case, e.g., for the calculation of photoabsorption spectra. (b) For a "strong" external potential, a full solution of the time-dependent Kohn-Sham equations is in order. This situation is encountered, for instance, when matter interacts with intense laser fields. In this review we give an overview of TDDFT from its theoretical foundations to several applications both in the linear and in the nonlinear regime.

1,283 citations