Search results for "Density Functional Theory"

A first-principles time-dependent density functional theory framework for spin and time-resolved angular-resolved photoelectron spectroscopy in perio…

2017

We present a novel theoretical approach to simulate spin, time, and angular-resolved photoelectron spectroscopy (ARPES) from first-principles that is applicable to surfaces, thin films, few layer systems, and low-dimensional nanostructures. The method is based on a general formulation in the framework of time-dependent density functional theory (TDDFT) to describe the real time-evolution of electrons escaping from a surface under the effect of any external (arbitrary) laser field. By extending the so-called t-SURFF method to periodic systems one can calculate the final photoelectron spectrum by collecting the flux of the ionization current trough an analyzing surface. The resulting approach…

Symmetry-induced long-lived excited state inAu6−

2007

We present time-resolved photoelectron spectra in combination with quantum chemical calculations based on time-dependent density functional theory for the study of the long-lived excited state of ${\mathrm{Au}}_{6}^{\ensuremath{-}}$. The experimental spectra indicate an excited state lifetime of at least $90\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. It is shown that the orbital symmetry of the photoexcited state as well as the planarity of ${\mathrm{Au}}_{6}^{\ensuremath{-}}$ are responsible for the unusual long lifetime. A possible decay mechanism is the fluorescence of a photon which is estimated to take place on a time scale of $730\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$.

2014

N-type CdO is a transparent conducting oxide (TCO) which has promise in a number of areas including solar cell applications. In order to realize this potential a detailed knowledge of the electronic structure of the material is essential. In particular, standard density functional theory (DFT) methods struggle to accurately predict fundamental material properties such as the band gap. This is largely due to the underestimation of the Cd 4d binding energy, which results in a strong hybridization with the valence-band (VB) states. In order to test theoretical approaches, comparisons to experiment need to be made. Here, synchrotron-radiation photoelectron spectroscopy (SR-PES) measurements are…

Method specific Cholesky decomposition : Coulomb and exchange energies

2008

We present a novel approach to the calculation of the Coulomb and exchange contributions to the total electronic energy in self consistent field and density functional theory. The numerical procedure is based on the Cholesky decomposition and involves decomposition of specific Hadamard product matrices that enter the energy expression. In this way, we determine an auxiliary basis and obtain a dramatic reduction in size as compared to the resolution of identity (RI) method. Although the auxiliary basis is determined from the energy expression, we have complete control of the errors in the gradient or Fock matrix. Another important advantage of this method specific Cholesky decomposition is t…

Comment on “Critique of the foundations of time-dependent density-functional theory”

2008

A recent paper [J. Schirmer and A. Dreuw, Phys. Rev A. 75, 022513 (2007)] challenges exact time-dependent density-functional theory (TDDFT) on several grounds. We explain why these criticisms are either irrelevant or incorrect, and that TDDFT is both formally exact and predictive.

Exchange-correlation potential with a proper long-range behavior for harmonically confined electron droplets

2010

The exchange-correlation potentials stemming from the local-density approximation and several generalized-gradient approximations are known to have incorrect asymptotic decay. This failure is independent of the dimensionality but so far the problem has been corrected---within the mentioned approximations---only in three dimensions. Here we provide a cured exchange-correlation potential for two-dimensional harmonically confined systems that cover a wide range of applications in quantum Hall and semiconductor physics, especially in quantum-dot modeling. The given potential is a generalized-gradient approximation and we demonstrate that it agrees very well with the analytic result of a two-ele…

Heating ofAl13−andAl14clusters

2001

Dynamical properties of ${\mathrm{Al}}_{13}^{\ensuremath{-}}$ and ${\mathrm{Al}}_{14}$ clusters at a high-temperature regime are studied using a density functional theory based first-principles simulations method. During the heating ${\mathrm{Al}}_{13}^{\ensuremath{-}}$ shows a significantly different behavior than ${\mathrm{Al}}_{14}$ due to its double-magic nature. We also demonstrate that it is hard to assign any distinct melting transition for the studied cluster sizes. For ${\mathrm{Al}}_{13}^{\ensuremath{-}}$ we observe a solidlike behavior well after the melting temperature of bulk aluminum. In contradiction with the rare gas clusters we notice that the outermost atom of icosahedral …

Semi-local density functional for the exchange-correlation energy of electrons in two dimensions

2010

We present a practical and accurate density functional for the exchange-correlation energy of electrons in two dimensions. The exchange part is based on a recent two-dimensional generalized-gradient approximation derived by considering the limits of small and large density gradients. The fully local correlation part is constructed following the Colle-Salvetti scheme and a Gaussian approximation for the pair density. The combination of these expressions is shown to provide an efficient density functional to calculate the total energies of two-dimensional electron systems such as semiconductor quantum dots. Excellent performance of the functional with respect to numerically exact reference da…

First-principles simulations for attosecond photoelectron spectroscopy based on time-dependent density functional theory

2018

We develop a first-principles simulation method for attosecond time-resolved photoelectron spectroscopy. This method enables us to directly simulate the whole experimental processes, including excitation, emission and detection on equal footing. To examine the performance of the method, we use it to compute the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) experiments of gas-phase Argon. The computed RABBITT photoionization delay is in very good agreement with recent experimental results from [Klünder et al., Phys. Rev. Lett. 106, 143002 (2011)] and [Guénot et al., Phys. Rev. A 85, 053424 (2012)]. This indicates the significance of a fully-consiste…

Exchange and correlation energy functionals for two-dimensional open-shell systems

2009

We consider density functionals for exchange and correlation energies in two-dimensional systems. The functionals are constructed by making use of exact constraints for the angular averages of the corresponding exchange and correlation holes, respectively, and assuming proportionality between their characteristic sizes. The electron current and spin are explicitly taken into account, so that the resulting functionals are suitable to deal with systems exhibiting orbital currents and/or spin polarization. Our numerical results show that in finite systems the proposed functionals outperform the standard two-dimensional local spin-density approximation, still performing well also in the importa…