Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U32: Density Functional Theory |
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Sponsoring Units: DCOMP Chair: Dimitrios Papaconstantopoulos, NRL Room: LACC 507 |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U32.00001: Distorted plane waves - new basis set for the interstitial region Fredrik Bultmark In the LAPW method plane waves are used to describe the wave function and charge density in the interstitial region between the muffin tin (MT) spheres. Plane waves are an excellent basis set for the interstitial region in many aspects - they have a well defined energy and simulate well the charge density of many materials. However in many applications the number of basis functions needed to describe the wave functions and charge density to desired precision is quite large and the diagonalisation of the corresponding Hamiltonian will be time consuming. In order to speed up the diagonalisation and make it possible to perform calculations on larger systems we implement a new set of basis functions, the distorted plane waves which conserves the simplicity of the plane waves, but reduce the number of basis functions needed to describe the wavefunctions to desired precision. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U32.00002: Time-dependent Kohn-Sham theory with memory Harshani O. Wijewardane, Carsten A. Ullrich In time-dependent density-functional theory, exchange and correlation (xc) beyond the adiabatic local density approximation can be described in terms of viscoelastic stresses in the electron liquid. In the time domain, this leads to a velocity-dependent xc vector potential with a memory containing short- and long-range components. The resulting time-dependent Kohn-Sham formalism describes the dynamics of electronic systems including decoherence and relaxation. For the example of collective charge-density oscillations in a quantum well, we illustrate the xc memory effects, clarify the dissipation mechanism, extract intersubband relaxation rates for weak and strong excitations, and demonstrate the generation of plasmon sidebands. This work was supported by the ACS Petroleum Research Fund and Research Corporation. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U32.00003: Test of a nonempirical density functional for short-range van der Waals interaction in rare-gas dimers Jianmin Tao, John Perdew It is known that the nonempirical generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) provides a much more realistic description of the short-range part of the van der Waals (vdW) interaction than does the local spin density (LSD) approximation. In the present work, the ability of the higher-level nonempirical meta-GGA of Tao, Perdew, Staroverov, and Scuseria (TPSS) [Phys. Rev. Lett. {\bf 91}, 146401 (2003)] to describe vdW interaction is tested self-consistently in rare-gas dimers with $Z \le 36$. The one-parameter hybrid version (TPSSh) of the TPSS exchange-correlation functional is also included in this test. Calculations show that both TPSS and TPSSh functionals correctly yield vdW bonds in these dimers and significantly improve the prediction of bond lengths and binding energies over LSD. The rather close agreement of TPSS with PBE for these dimers confirms a principle of the TPSS construction: preservation of the PBE large-gradient behavior. Compared with the PBE GGA, TPSS and TPSSh yield a slightly weaker binding. The typically too-long bond lengths and too-small binding energies of TPSS meta-GGA suggest the need for some long-range vdW interaction correction which is discussed. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U32.00004: Analytical Solutions for States of the 3D Hooke's Atom in an External B Field Samuel Trickey, Wuming Zhu Hooke's atom is a 2-electron model with the nuclear-electron attraction replaced by an isotropic harmonic potential. Closed-form solutions for certain eigenstates are known in 3D at B=0; B $\ne$ 0 solutions are known only for the 2D quantum dot. Both solutions are products of center-of-mass oscillators and relative motion factors. Because the uniform-B confining potential is quadratic in the cartesian coordinates normal to B, we can find related analytical solutions for certain eignvalues of the 3D Hooke's atom at B $\ne$ 0. They are more complicated because of the imposed axial symmetry. We sketch the somewhat tedious solution techniques, then compare the analytical and numerical solutions for a large range of field strengths. We have used these results to obtain exact Kohn-Sham orbitals for current density functional theory (CDFT) [``Exact Current DFT Study of Hooke's Atom in Magnetic Fields,'' W. Zhu and S.B. Trickey to be published] [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U32.00005: Exact Current DFT Study of Hooke's Atom in Magnetic Fields Wuming Zhu, Samuel Trickey From exact analytical [1] and numerical solutions for Hooke's atom in a uniform external magnetic field, we construct the exact Kohn-Sham (KS)orbitals for current density functional theory (CDFT). We discuss the effects of the external B field relative to the harmonic confining potential on the exchange-correlation energy and various energy components, as well as exact exchange-correlation scalar and vector potentials. Exact density functional results are compared with results with several widely used approximate DFT functionals. Our exact CDFT results can be used as a check for any proposed CDFT functionals and as guidance for improvement of existing functionals. [1] "Analytical Solutions for States of the 3D Hooke's Atom in an External B Field", W. Zhu and S.B. Trickey to be published. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U32.00006: Exact-exchange density-functional calculations for large gap materials: A major step forward? Rudolph Magyar, Andrzej Fleszar, Eberhard Gross The electronic structure of several large gap insulators is calculated using the exact-exchange functional (EXX) in density functional theory, and the results are compared with those from the local-density approximation (LDA) and experiment. EXX is considered a major step beyond LDA and has already been shown to provide exceptionally accurate results for semi-conductors. In this study, two classes of large gap systems are examined, the noble-gas solids and simple biatomic ionic crystals. For the noble-gas solids, the dominant binding effect is the Van der Waals interaction which is not properly described by the EXX formalism. Ionic crystals, instead, are held together by the Hartree interaction between oppositely charged ions, and the Van der Waals interaction plays a negligible role. It is seen that the EXX method does not reproduce the fundamental energy gaps as well as has been reported for semiconductors; however, the EXX gaps are much closer to the optical gaps than LDA gaps and still represents a significant advance in band theory calculations. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U32.00007: Relaxation and dissipation in time-dependent current-density functional theory Roberto D'Agosta, Giovanni Vignale In a typical relaxation problem a many-particle system evolves from an initial excited state under the action of its own hamiltonian plus a ``thermal bath", until equilibrium (or the ground-state at $T=0$) is reached. Due to the presence of the thermal bath the time evolution of the system is not unitary, and an initially pure state will evolve into a statistical mixture of states. Here we show that the time-dependent current density functional theory$^1$ allows a hamiltonian description of the relaxation process, whereby the quantum state of the system undergoes a unitary time evolution without becoming entangled with a thermal bath. The essential feature that causes the system to eventually settle into a stationary state of the ground-state Kohn-Sham hamiltonian is the presence of an effective electric field, which is determined by the instantaneous values of the current and the density. Our theory is consistent with recent numerical results by Wijewardane and Ullrich$^ 2$.\\ 1. G. Vignale, C. A. Ullrich, and S. Conti, PRL {\bf 79}, 4878 (1997)\\ 2. H. O. Wijewardane and C. A. Ullrich, cond-mat/0411157 [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U32.00008: Correlation Effects in Screened-Exchange Density Functional Theory Byounghak Lee, Lin-Wang Wang While it has been demonstrated that the screened-exchange local density approximation (sX-LDA) gives good agreement with experiment for fundamental energy gaps of many semiconducting systems, the underlying physics is not always clear. One particular question is, in semiconductor systems, whether the screening should be short range (e.g., the Thomas-Fermi screening) or long range (e.g., by the semiconductor dielectric function). To investigate this, we have compared the self-energy term in the sX-LDA formalism with the self-energy term in the GW approximation and the exchange-correlation hole of variational quantum Monte Carlo simulations. We have also tested the band gaps and total energy results within the sX-LDA formalism with different screening models. The sX-LDA calculations are done using norm-conserving pseudopotentials and a plane-wave basis. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U32.00009: Building improved functionals for self-consistent DFT by better treatment of electronic surface regions Rickard Armiento, Ann E. Mattsson We develop a specialized treatment of electronic surface regions which, via the subsystem functional approach [1], can be used in functionals for self-consistent density-functional theory (DFT). Approximations for both exchange and correlation energies are derived for an electronic surface. An interpolation index is used to combine this surface-specific functional with a functional for interior regions. When the local density approximation (LDA) is used for the interior region, the end result is a straightforward density-gradient dependent functional that shows promising results. Further improvement of the treatment of the interior region by the use of a local gradient expansion approximation is also discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] R. Armiento and A. E. Mattsson, Phys. Rev. B {\bf 66}, 165117 (2002). [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U32.00010: A functional designed to include surface effects into self-consistent density-functional theory calculations. A.E. Mattsson, R. Armiento We present an exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. The functional is developed within the subsystem functional paradigm [1], combining the local density approximation for interior regions with a new functional designed for surface regions. It is validated for a variety of materials by calculations of: (i) properties where surface effects exist, and (ii) established bulk properties. Good and coherent results are obtained, indicating that this functional may serve well as universal first choice for solid state systems. The good performance of this first subsystem functional also suggests that yet improved functionals can be constructed by this approach. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [1] R.~Armiento and A.~E.~Mattsson, Phys.~Rev. B, {\bf 66}, 165117 (2002) [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U32.00011: Empirical Laplacian-based model of the exchange-correlation energy density and potential in Si A.C. Cancio, M.Y. Chou We explore density functional theory (DFT) models based on the Laplacian of the local density, derived from a fit to accurate variational Monte Carlo (VMC) data for the adiabatic exchange-correlation energy density of the Si crystal. We find that the rms discrepancy between the local density approximation (LDA) and the VMC data is reduced 70\% using a three-parameter correction to the LDA that incorporates the local Laplacian only, with a similar reduction in error for the Si atom with no modification to the model. Corrections to the LDA exchange-correlation potential generated by Laplacian terms have been implemented within a pseudopotential plane-wave scheme. Self-consistent calculations of the structural properties of Si using this potential reproduce those of the LDA. In contrast, the local gradient of the density provides an insignificant improvement to the fit, while introducing unphysical features into the exchange-correlation potential, and giving a significantly poorer description of structural properties. Application of our model to other semiconductors will be briefly discussed. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U32.00012: Magnesium Alloy Precipitate Formation Using Mixed Basis Cluster Expansions Robert Leone, Gus Hart Unlike steel and aluminum alloys, the basic science of magnesium alloys is poorly understood. The automotive industry is driving demand for lighter structural material, and readily available magnesium alloys have a higher strength-to-weight ratio than their aluminum counterparts. We seek to predict magnesium alloy properties from first principles, particularly the hardening effect of precipitate formation. Mixed basis cluster expansions (MBCE) have successfully modeled precipitate shapes and growth in aluminum alloys. Unfortunately, this methodology has not been extended to hcp-based materials such as magnesium alloys. In order to model binary magnesium alloys using the MBCE, particularly precipitate morphologies, we have constructed a coherency strain model for hcp structures to correctly represent the long range strain fields around precipitates. Coupling this generalized strain model to an Ising-like expansion methodology we have developed a mixed-basis cluster expansion for hexagonal symmetries. Results for several representative magnesium alloys will be presented. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U32.00013: Self-Consistent Radiative Scattering in Time Dependent Density Functional Theory Ryan Hatcher, Alan Tackett, Sokrates Pantelides Time Dependent Density Functional Theory (TDDFT) is an ab initio theory that can be used to model time varying electron densities. We propose a semi-classical method for calculating the self-consistent radiation from a time varying electron density in a TDDFT framework. This scheme allows one to simulate a system where a time varying electron density scatters energy both into the lattice as well as into an electro-magnetic field. We will present a description of this technique and describe a few applications in which it has been employed. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U32.00014: Many-body perturbation theory using the density-functional concept: beyond the GW approximation Fabien Bruneval, Francesco Sottile, Valerio Olevano, Rodolfo Del Sole, Lucia Reining Electronic structure calculations based on the Many-Body Perturbation Theory (MBPT), within GW approximation [1], are very reliable for simple materials. Some failures (e.g. for transition metals) ask for better approximations that could be in principle derived through the usual set of MBPT equations. This procedure would be however computationally prohibitive. We propose an alternative formulation of MBPT that relies on density-functional concept [2]. Our equation for the polarizability is a two-point one, that leads to excellent optical absorption and energy loss spectra [3]. The other MBPT quantities are then simply calculated via an integration. The terms beyond GW approximations, that we obtain, are both more simple and more physically intuitive, than the usual ones. We show a direct impact of this formulation on the TDDFT. Numerical results for optical absorption, IXS, and the band gap of bulk silicon and solid argon illustrate the leading corrections beyond RPA for the polarizability and GW for the self-energy. [1] L. Hedin, Phys. Rev. {\bf 139}, A796 (1965). [2] F. Bruneval {\it et al.}, submitted to Phys. Rev. Lett. [3] F. Sottile, V. Olevano, and L. Reining, Phys. Rev. Lett. {\bf 91}, 056402 (2004). [Preview Abstract] |
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U32.00015: A new approach to the treatment of uniform electric fields Roberto D'Agosta, Giovanni Vignale It has been known for a long time that the treatment of an external uniform electric field in a periodic system presents conceptual and practical difficulties. At the heart of these difficulties lies the fact that, when a uniform electric field is present, the ground state does not exist: thus one is faced with the dilemma of either breaking the periodicity by a scalar potential or making the problem time-dependent by a vector potential. Within the context of density functional theory this seems to imply that the conventional description of many-body effects in terms of the time-dependent density should be abandoned in favor of a description in terms of the current density.$^1$ However, we will show that it is possible to describe the uniform electric field without leaving the framework of ordinary time-dependent DFT, by passing to a non-inertial reference frame. By leaving the distances invariant, this transformation preserves the periodicity of the lattice, and at the same time the appearance of an ``inertial force" compensates for the vector potential. Thus, we end up with a system subjected to a periodic time-dependent external potential -- a perfectly legitimate candidate for the application of TDDFT.\\ 1. N.T. Maitra, I. Souza, and K. Burke, Phys. Rev. B. {\bf 68}, 045019, (2003). [Preview Abstract] |
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