Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L19: Frontiers in Electronic Structure Theory II |
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Sponsoring Units: DCP DCOMP Chair: Garret Chan, Cornell University Room: Colorado Convention Center 104 |
Tuesday, March 6, 2007 2:30PM - 3:06PM |
L19.00001: Many-Body Perturbation Theory and Density-Functional based approaches: successful combinations Invited Speaker: Today, in the framework of solid state physics two main ab initio approaches are used to describe ground- and excited state properties of condensed matter: on one side, static ground state density functional theory (DFT) and its time-dependent extension (TDDFT) for the description of excited states; on the other side, Many-Boby Perturbation Theory (MBPT), most often used in Hedin s GW approximation [1] for the electron self-energy, or the Bethe-Salpeter equation for the calculation of response functions. Both approaches have led to breakthroughs, but suffer from different shortcomings: MBPT has a relative conceptual clarity and therefore allows one to find good approximations, but calculations are in general numerically very demanding. DFT-based approaches are in principle computationally more efficient, but a generally reliable and at the same time efficient description of exchange-correlation effects within TDDFT is difficult to obtain. In recent years a major effort has therefore been made in order to combine MBPT and TDDFT, searching for a formulation that would keep the advantages of both approaches (see e.g. [2,3]). In this talk we will discuss different ways to derive a linear response exchange-correlation kernel for TDDFT from MBPT. The strength of various approximations, that have been shown to reproduce continuum and bound excitons for a wide range of materials, as well as possible problems will be outlined, and the computational efficiency of the method examined. The question of how to use such a combination of MBPT and TDDFT in order to obtain vertex corrections to the self-energy [4] will also be addressed. \newline \newline [1] L. Hedin, Phys. Rev. 139, A796 (1965). \newline [2]F. Sottile, V. Olevano, and L. Reining, Phys. Rev. Lett. 91, 056402 (2003) \newline [3] S. Botti, F. Sottile, N. Vast, V. Olevano, L. Reining, H.-C. Weissker, A. Rubio, G. Onida, R. Del Sole, R.W. Godby, Phys. Rev. 69, 155112, (2004). \newline [4] F. Bruneval, F. Sottile, V. Olevano, R. Del Sole and L. Reining, Phys. Rev. Lett. 94, 186402, (2005). [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:42PM |
L19.00002: Using Constrained DFT to Define a Diabatic Configuration Space Invited Speaker: We show that several of the well-known shortcomings of approximate density functionals for treating electron transfer (ET) can be overcome by applying physically motivated constraints to the electron density. We summarize our implementation of this constrained density functional theory (CDFT) and present several illustrative applications that demonstrate the strengths of the new formalism: 1) CDFT allows charge transfer excitations to be treated accurately within a ground state formalism, including the long range -1/r interaction between the electron and the hole 2) One directly obtains diabatic states, which can be unambiguously associated with Marcus theory parameters and 3) Long-standing ground state electronic structure problems -- such as the prediction of exchange couplings and certain reaction barrier heights -- can be treated accurately in a rigorous fashion. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L19.00003: Ab initio study of near-edge x-ray absorption fine structure of hexagonal ice and liquid water Wei Chen, Roberto Car We report first-principles calculations of near-edge x-ray absorption fine structure (NEXAFS) spectra of hexagonal ice and liquid water. Our work is motivated by the importance of accurately modeling NEXAFS spectra, which provide sensitive information on local molecular structures. In particular, we find a systematic improvement in the agreement of the calculated spectra with the experiment, by including excitonic effects, final state and self-interaction corrections. We correlate the calculated corrections to the degree of localization of the excited states. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L19.00004: Going beyond the Tamm-Dancoff approximation in the Bethe-Salpeter approach to the optical properties of solids Peter Puschnig, Claudia Ambrosch-Draxl The solution of the Bethe-Salpeter equation (BSE) has turned out to be the method of choice for the ab-initio calculation of optical properties of semiconductors and insulators which is capable of correctly accounting for excitonic effects. Commonly, however, the coupling between the resonant and anti-resonant excitations is neglected, referred to as the Tamm-Dancoff approximation (TDA). This is well justified in many cases, in particular, for the working horses of theoretical solid state physics, such as bulk Si and GaAs. Here, we report on a first-principles investigation of the optical properties of organic semiconductors which are highly anisotropic systems. We find that the TDA no longer holds in such low-dimensional systems, where the exciton binding energies are no longer small compared to the band gaps. Going beyond the TDA leads to an increase of the exciton binding energy in the order of several tenths of an eV thereby considerably improving the agreement with experiment. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L19.00005: Electron-atom scattering using time-dependent density-functional theory Meta van Faassen, Kieron Burke We present a method to obtain single-channel elastic electron-atom scattering phase shifts from time-dependent density functional theory (TDDFT). The system is placed in a spherical box, and TDDFT is used to calculate its discrete spectrum, from which phase shifts are deduced. The influence of ground state Kohn-Sham potentials and exchange-correlation kernels on the results are discussed. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L19.00006: Investigation of Bonding in the BF$_{3}$-H$_{2}$O Complex Archana Dubey, H.P. Saha, Lee Chow, R.H. Pink, Dip N. Mahato, M.B. Huang, T.P. Das, R.H. Scheicher, Mahendra K. Mahanti The catalytic properties of BF$_{3}$ involving its complexes with different classes of molecules is of great current interest. As a typical system of complexes involving the B-O bond we have studied the BF$_{3}$- H$_{2}$O system using first-principle Hartree-Fock-Roothaan procedure combined with many-body perturbation theory to include Van der Waals (VDW) interaction between BF$_{3}$ and H$_{2}$O molecules. From our results, the VDW contribution to the binding energy of the BF$_{3}$-H$_{2}$O complex comes out as 34.5{\%} of the covalency, close to the 36.4{\%} result from our earlier investigations on BF$_{3}$-NH$_{3}$ . The absolute values for the covalency and VDW contributions are both about 35{\%} of the BF$_{3}$-NH$_{3 }$result. Physical implications of these results will be discussed. [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L19.00007: van der Waals coefficients in DFT: a simple approximation for the polarizability Stefano de Gironcoli, Huy Viet Nguyen Long range van der Waals interaction plays a crucial role in many systems. Density functional Theory (DFT) within Local Density and Generalized Gradient Approximations for exchange-correlation energy is known to fail in describing properly this interaction, while direct calculations based on the exact Adiabatic Connection Formula are computationally impracticable, except for few simple systems. A simple and computationally fast scheme to calculate imaginary-frequency-dependent polarizability, hence asymptotic van der Waals interaction, within density functional theory is considered. The van der Waals coefficients for a large number of closed-shell ions and several molecules are calculated and compare well with available values obtained by more refined first-principle calculations. The success in these test cases shows the potential of the approximation in capturing the essence of long range correlations and may give useful information for constructing a functional which naturally includes van der Waals interaction in DFT. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L19.00008: First-Principles Study of the Nature of Binding in BF$_{3}$ Molecular Solids Dip N. Mahato, R.H. Pink, M.B. Huang, T.P. Das, Archana Dubey, Lee Chow, Mahendra K. Mahanti, R. H. Scheicher The binding of BF$_{3}$ molecules in solid BF$_{3}$ is studied by the Hartree-Fock Cluster Procedure, with Van der Waals interaction between the BF$_{3}$ molecules included by the many-body perturbation theory procedure. The binding appears to be the result of strong cancellation between one-electron effects, represented by the covalent interaction between neighboring molecules combined with the coulomb interaction between the effective charges on the boron and fluorines in each of the neutral BF$_{3}$ molecules and the many-body correlation effect between electrons on neighboring molecules leading to the Van der Waals interaction, the latter being the determining factor for the binding. Quantitative results will be presented for the binding energy in this solid state system which represents a class of molecular solids for which the neutral molecular units have substantial effective charges of different signs on the constituent atoms.$_{ }$ [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L19.00009: Optimized orbitals with second order opposite-spin correlation Martin Head-Gordon Despite tremendous progress, the most ubiquitous electronic structure methods, based on density functional theory (DFT), that can be applied to molecules ranging well over 100 atoms, exhibit failures for molecules with strong correlations, some types of radicals, and systems where dispersion interactions are important. At the same time, the most accurate electronic structure methods, based on coupled cluster theory, remain too computationally demanding to enable the routine treatment of molecules containing more than about 20 atoms. I will discuss a new self-consistent approach that correctly and inexpensively recovers dispersion interactions, without either excessive spin-contamination for radicals (as plagues traditional unrestricted Hartree-Fock-based methods), or the difficulties of self-interaction that can affect DFT calculations of radicals. This approach yields optimized Breuckner-type orbitals. Its performance for relative energies, structures, and frequencies will be assessed, both for closed shell molecules, radicals, as well as some cases which exhibit pathological failures at both the DFT and MP2 levels of theory. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L19.00010: On the dynamics of the spin-boson model: variational principle versus adiabatic approximation Titus Sandu, Radu Iftimie The time-dependent variational principle is applied to the spin-boson model. We use two different trial functions that exhibit various degrees of separation between the bosonic dynamics and the electronic dynamics. The equations of motion obtained for these two trial functions are shown to be equivalent with the equations of motions obtained with two different adiabatic approximations of the dynamics. [Preview Abstract] |
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