Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B37: Focus Session: Fundamental Developments in Density Functional Theory II |
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Sponsoring Units: DCP Chair: Paula Mori-Sanchez, Duke University Room: 409 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B37.00001: Phase-Space Explorations in Time-Dependent Density Functional Theory Invited Speaker: Neepa Maitra Time-Dependent Density Functional Theory is increasingly popular for calculating excitation and response properties of atoms, molecules, clusters and solids. It has achieved an unprecedented balance between accuracy and efficiency for a wide range of systems, but not all. Although not limited to the linear response regime, there are particular challenges for applications to strongfield processes; for example obtaining momentum distributions, certain electronic quantum control problems, and including memory-dependence necessary in the functional dependence. In this talk we will discuss some of these, and introduce a new extension of the theory where the basic variable is the phase-space density W(r,p,t) (that contains information on both the co-ordinate- and momentum distributions of the electrons), instead of the usual co-ordinate space density n(r,t), to deal with these challenges. [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:27PM |
B37.00002: Time-dependent density-functional theory for electronic excitations in materials Invited Speaker: Carsten A. Ullrich There is currently an intense effort underway to study the optical properties of bulk and nanostructured materials using time-dependent density-functional theory (TDDFT). This talk will discuss challenges and recent advances of TDDFT in this area, and present some new applications to excitonic effects in bulk insulators and to collective charge- and spin-density excitations in doped quantum wells. A TDDFT version of the semiconductor Bloch equations is presented, which describes ultrafast electron dynamics, including excitonic effects, in insulators and semiconductors. From this, an excitonic Wannier equation is derived featuring a nonlocal effective electron-hole interaction determined by long-range exchange-correlation effects. Excitonic binding energies are calculated for several direct-gap insulators. The spin Coulomb drag (SCD), which constitutes an intrinsic source of dissipation for spin currents in metals and semiconductors, originates as a dynamical exchange-correlation effect in time-dependent current-DFT. We develop a linear-response description of collective spin-density excitations in quantum wells including SCD as well as Rashba and Dresselhaus spin-orbit coupling, and show that spin plasmon line widths in quantum wells allow a purely optical, quantitative measurement of the SCD effect. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B37.00003: Many-Pole Self-Energy Model Corrections to Kohn-Sham Calculations of Excited State Spectra J. J. Kas, M. Prange, J. J. Rehr, H. M. Lawler Experimental x-ray spectra are systematically shifted and broadened with respect to conventional density functional theory calculations due to photoelectron self-energy effects. We have recently developed an efficient many-pole model of the GW self-energy based calculations of dielectric responce using a real-space Green's function approach.\footnote{J.J. Kas et al., Phys. Rev. B {\bf 76}, 195116 (2007).} The model is applied \textit{a posteriori} to Kohn-Sham calculations of excited state spectra using a convolution of the spectrum with an energy dependent Lorenzian. The method is found to be widely applicable over a broad range of energies, with little computational cost. Several illustrative examples are presented which show improved agreement between theoretical calculations and experiment for both optical and x-ray spectra. [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B37.00004: First principle calculations of long range correlation effects within the random phase approximation Deyu Lu, Yan Li, Hugh Wilson, Giulia Galli The local and semi-local approximations to Density Functional Theory fail to describe correctly certain types of weak interactions (e.g. van der Waals forces) due an incorrect account of long range correlation effects. Such effects may be described by computing correlation energies within the random phase approximation (RPA), using the fluctuation-dissipation theorem and the adiabatic connection. We present an approach to compute RPA correlation energies based on an eigenmode expansion of the dielectric matrix [1,2]. By solving the frequency dependent Sternheimer equation within linear response theory [3], we eliminate the need to compute single particle unoccupied states, which makes our approach more efficient than methods using the direct-summation technique. Furthermore, the use of a dielectric eigenmode representation allows for a physical interpretation of several, different contributions to correlation energies. Results for graphite and the benzene crystal will be discussed. [1] H. Wilson, F. Gygi and G. Galli, Phys. Rev. B, 78:113303, (2008). [2] D. Lu, F. Gygi and G. Galli, Phys. Rev. Lett., 100:147601(2008). [3] S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Rev. Mod. Phys. 73:515, (2001). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B37.00005: Tailoring High-Order Harmonics: A Computational Approach Based on Time-Dependent Density-Functional Theory Alberto Castro, Ali Akbari, Angel Rubio, Eberhard Gross Atoms and molecules react in complex manners when they are irradiated with high-intensity electromagnetic pulses: multi-photon, tunnelling and over-the-barrier ionisation, laser driven photo-induced isomerisations or fragmentations, and high harmonic generation are some of the non-linear effects that are observed. The so-called pulse shaping techniques can be used to design pulses that produce a desired effect. A technologically appealing possibility is to tailor the harmonic emission spectrum: enhancement of some given orders, supressions of others, etc. We have undertaken the task of exploring this possibility from a theoretical point of view, by making use of time-dependent density-functional theory to describe the electrons, a real-space numerical representation, and various optimization techniques. [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B37.00006: Ab initio calculations of optical spectra by solving the Bethe-Salpeter equation without empty states.Work Dario Rocca, Deyu Lu, Giulia Galli We present a novel first principle approach to solve the Bethe-Salpeter equation (BSE) that builds on recent progress in time-dependent density functional perturbation theory [1], and uses an eigenvalue decomposition representation of the dielectric matrix [2]. This approach does not require the explicit calculation of excited single particle electronic states, making it suitable for calculations involving large basis sets and/or a large number of transitions. The numerical solution of the BSE is obtained through a generalized, non-Hermitian Lanczos iterative algorithm and does not require the use of the Tamm-Dancoff approximation. Furthermore, since Lanczos coefficients are frequency independent, optical spectra may be obtained in a very broad energy range. The efficiency and accuracy of the new approach are demonstrated by calculating the optical properties of silicon nanoclusters with up to 1 nm diameter. [1] D. Rocca, R. Gebauer, Y Saad, and S. Baroni, J. Chem. Phys. 128, 154105 (2008). [2] H.Wilson, F.Gygi and G.Galli, Phys. Rev. B 78, 113303 (2008). [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B37.00007: Time-dependent transition densities Yonghui Li, Carsten A. Ullrich To visualize and interpret the induced charges and electron-hole coherences of electronic excitations in molecules, a real-space density matrix analysis is a useful computational tool. We extend this technique into the nonlinear, real-time domain and define the time-dependent transition densities in the context of time-dependent density-functional theory. This opens up the possibility of a real-time monitoring of the optical excitation dynamics in molecules, providing a visualization tool for processes such as exciton migration or charge-transfer excitations. The method will be illustrated for simple one-dimensional model systems. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B37.00008: Is the Kohn-Sham Oscillator Strength Exact at the Ionization Threshold? Zenghui Yang, Meta van Faassen, Kieron Burke It is well-established that the highest occupied orbital of the exact Kohn-Sham potential of any system is at -I, where I is the ionization energy. Therefore, in optical response, the non-interacting Kohn-Sham electrons in the ground-state potential have a first ionization threshold that exactly matches that of the real system[1]. We show that corresponding the Kohn-Sham oscillator strength is not exact at the first ionization threshold by explicit demonstration for the helium atom. We use a simple fit of the entire photoabsorption spectrum of both the Kohn-Sham potential for helium and that of real helium. We use oscillator strength sum rules[2] to determine the fit parameters, so this fit should be generally useful. [1] M. A. L. Marques, C. A. Ullrich, F. Nogueira, et al. Time-Dependent Density Functional Theory. Springer-Verlag, Berlin, 2006 [2] U. Fano and J. W. Cooper. Rev. Mod. Phys., 40(3), 441-507, 1968 [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B37.00009: An Exact Condition for the Integrand of Adiabatic Connection Zhenfei Liu, Kieron Burke In density functional theory (DFT), the exchange-correlation functional $E_{\rm XC}$ can be exactly expressed by the adiabatic connection integral [1,2]. The integrand should satisfy several exact conditions [3]. We show that for the low-density limit (as $\lambda \to \infty$), the $\lambda^{-1}$ term in the expansion of the integrand $W(\lambda)$, should vanish. We propose a simple parametric form for $W(\lambda)$, satisfying the new exact condition. We apply this interpolation form to Hooke's atom and helium atom and show that it is accurate for weakly-correlated two-electron systems. \\[3pt] [1] D.C. Langreth and J.P. Perdew, Solid State Commun. {\bf 17}, 1425 (1975). \\[0pt] [2] O. Gunnarsson and B.I. Lundqvist, Phys. Rev. B {\bf 13}, 4274 (1976). \\[0pt] [3] M. Seidl, J.P. Perdew and M. Levy, Phys. Rev. A {\bf 59}, 51 (1999). [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B37.00010: New exact and approximate forms for the Luttinger-Ward correlation energy functional within the GW-RPA approximation Sohrab Ismail-Beigi In principle, many-body Green's function approaches to electronic systems such as the Luttinger-Ward formalism allow one to compute both total energies and quasiparticle excitation spectra (i.e. band structures) simultaneously from first principles. We report on two new results that reformulate the Luttinger-Ward correlation energy functional within the GW-RPA approximation. A first expression is exact and allows for systematic and straightforward evaluation of correlation energies. The second expression is approximate but yields a family of computationally efficient approximations to the correlation energy and the self-energy operator of which the well known Coulomb-hole and screened-exchange (COHSEX) approximation is the lowest order. [Preview Abstract] |
Monday, March 16, 2009 2:03PM - 2:15PM |
B37.00011: Non-empirical hyper-generalized-gradient functionals constructed from the Lieb-Oxford bound Mariana M. Odashima, Klaus Capelle A simple and completely general representation of the exact exchange-correlation functional of density-functional theory is derived from the universal Lieb-Oxford bound for Coulomb-interacting systems. This representation leads to an alternative point of view on popular hybrid functionals. A similar representation of the exact correlation functional allows to construct a family of non-empirical hyper-generalized-gradient approximations (HGGAs), departing from established paradigms of functional construction. Numerical tests and applications of these HGGAs to atoms and molecules demonstrate that even simple Lieb-Oxford based HGGAs are competitive with correlation functionals currently used in solid-state physics and quantum chemistry. [Preview Abstract] |
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