Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session N24: Electronic Structure Methods II |
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Sponsoring Units: DCOMP Chair: Bin Wang, Vanderbilt University Room: 326 |
Wednesday, March 20, 2013 11:15AM - 11:27AM |
N24.00001: Large-scale Bethe-Salpeter equation calculations of core-level x-ray spectra J. J. Rehr, J. Vinson, K. Gilmore Recently an approach has been developed for Bethe-Salpeter equation (BSE) calculations of core-level x-ray spectra, which is implemented in the \textsc{ocean} package \footnote{ J. Vinson, E. L. Shirley, J. J. Rehr, and J. J. Kas, Phys. Rev. B {\bf 83}, 115106 (2011); J. Vinson and J. J. Rehr, Phys. Rev. B (in press , 2012)} which combines plane-wave, pseudopotential DFT electronic structure, PAW transition elements, GW self-energy corrections, and the NIST BSE solver. The method yields both dipole limited and finite momentum transfer spectra. Here we discuss several recent advances which yield a unified treatment of both extended states and atomic multiplet effects. In particular our approach now includes spin-dependent potentials and hole-dependent lifetimes, and gives an improved treatment of L$_{2,3}$ edges, where contributions to spectral weight come from a mix of two distinct core holes. We have also extended the code interface to include pseudopotential wave functions from \textsc{abinit}, \textsc{QuantumEspresso}, or an interpolation based scheme, thus enabling large-scale calculations with unit cells in excess of 2000 \AA$^3$. Applications to water and ice structures are briefly discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:27AM - 11:39AM |
N24.00002: Calculation of charge-transfer satellites in x-ray absorption spectroscopy of transition metal oxides E. Klevak, J.J. Kas, J.J. Rehr Charge-transfer (CT) satellites in x-ray absorption spectroscopy (XAS) require treatments of correlation effects beyond the quasi-particle approximation. Here we present an approach for including CT effects in XAS that follows the model of Lee \textit{et al.}\footnote{J.D. Lee, O. Gunnarsson and L. Hedin, Phys. Rev. B \textbf{60}, 8034 (1999)} The approach is based on a three level system coupled to an itinerant state, with parameters obtained from either ab inito calculations or x-ray photoemission spectroscopy. The model yields an approximation to CT satellites in XAS in terms of a convolution of the quasi-particle spectrum with an energy-dependent spectral function that accounts for both localized CT excitations and solid state effects. The approach illustrates the crossover from the sudden to adiabatic approximations. Calculations for transition metal oxides, e.g. NiO and CoO, give reasonable agreement with XAS experiment. Finally, an extension of the present approach to CT satellites in resonant inelastic x-ray spectroscopy is also discussed. [Preview Abstract] |
Wednesday, March 20, 2013 11:39AM - 11:51AM |
N24.00003: Correlation matrix renormalization approximation for total energy calculations of correlated electron systems Y.X. Yao, C. Liu, J. Liu, W.C. Lu, C.Z. Wang, K.M. Ho The recently introduced correlation matrix renormalization approximation (CMRA) was further developed by adopting a completely factorizable form for the renormalization z-factors, which assumes the validity of the Wick's theorem with respect to Gutzwiller wave function. This approximation (CMR-II) shows better dissociation behavior than the original one (CMR-I) based on the straightforward generalization of the Gutzwiller approximation to two-body interactions. We further improved the performance of CMRA by redefining the z-factors as a function of f(z) in CMR-II, which we call CMR-III. We obtained an analytical expression of f(z) by enforcing the equality in energy functional between CMR-III and full configuration interaction for the benchmark minimal basis H2. We show that CMR-III yields quite good binding energies and dissociation behaviors for various hydrogen clusters with converged basis set. Finally, we apply CMR-III to hydrogen crystal phases and compare the results with quantum Monte Carlo. [Preview Abstract] |
Wednesday, March 20, 2013 11:51AM - 12:03PM |
N24.00004: Disorder effects in solid state systems beyond a single-site prospective: theories and applications Alberto Marmodoro, Arthur Ernst We review development and applications of improvement attempts upon the original Coherent Potential Approximation for the first-principles treatment of disordered systems. The single-site theory is examined in its basic aspects of analyticity and convergence, and compared with alternative methods for the study of solid state systems where a rigorous application of Bloch's theorem is no longer possible. The aspects of local environment effects, short-range ordering and off-diagonal disorder are considered in different extension proposals, in tight-binding and ab-initio illustrations based on multiple-scattering theory. In this context, results from application of a generalized version of the method are discussed evaluating some effects of disorder in solid state metallic solutions, molar doping materials for fuel cell technology, and magnetic compounds and excitations. Results from alternative methodologies such as supercell or special quasi-random structure approximations are also examined. [Preview Abstract] |
Wednesday, March 20, 2013 12:03PM - 12:15PM |
N24.00005: First-principles studies of photoelectron spectroscopy of solvated hydronium and hydroxide in water Charles Swartz, Xifan Wu Solvated hydronium (H3O$^+$) and hydroxide (OH$^-$) are important solutions of water defects. In a recent state-of-the-art photoelectron spectroscopy (PES) experiment, the binding energies of these water defects have been measured. Theoretically, we show that the photoelectron spectroscopy can be accurately computed based on GW quasi-particle theories, in which the molecular solvation structures are generated by ab initio molecular dynamics (AIMD). The resulting hydronium and hydroxide binding energies are 10 eV and 19 eV respectively, which are closely consistent with the recent PES experimental values of 9.2 eV and 20 eV. A close inspection reveals that the defect orbitals originate from the 1b$_2$ (1b$_1$) state of H3O$^+$ (OH$^-$) molecules in the gas phase. These orbitals are further strongly distorted by the surrounding water molecules, in which the H3O+ and OH- defects states are clearly localized on the so-called Zundel and Eigen solvation structures respectively. Proton transfers are found to further broaden the PES spectrum, which is more prominent in H3O$^+$ than in OH$^-$ solutions. [Preview Abstract] |
Wednesday, March 20, 2013 12:15PM - 12:27PM |
N24.00006: Bethe-Salpeter equation calculations of resonant inelastic x-ray scattering at the nitrogen K edge John Vinson, Terrence Jach, Tim Elam, Jonathon Denlinger We present theoretical calculations of resonant inelastic x-ray scattering (RIXS) at the nitrogen K edge of several materials along with direct comparison to experimental results. Our approach is based on a Bethe-Salpeter equation formalism, and our calculations are carried out using an extension of the \textsc{ocean} package,\footnote{J. Vinson, E. L. Shirley, J. J. Rehr, and J. J. Kas, Phys. Rev. B {\bf 83}, 115106 (2011) } including both intermediate and final-state excitonic effects. By building upon a DFT basis we include ground-state effects without system-dependent fitting parameters. We are able to account for the general trends and features seen in experiment. A more \textit{ad hoc} account of other contributions to the measured spectra, primarily phonon coupling, is attempted, but this highlights some current shortcomings limiting fully \textit{ab initio} calculations of the near-edge x-ray spectra of extended systems. [Preview Abstract] |
Wednesday, March 20, 2013 12:27PM - 12:39PM |
N24.00007: Generalized optimization of Wannier functions Emanuel Lazar, Hyowon Park, Chris Marianetti, Andy Millis Marzari and Vanderbilt introduced and developed a technique for defining and computing ``maximally localized'' Wannier functions to represent localized orbitals in periodic materials [1]. Since then, this method has been heavily used in computational condensed matter physics calculations. The Marzari-Vanderbilt procedure localizes all orbitals in a given energy window. In this talk we present some ongoing work in generalized minimization strategies which can apply different constraints to different subspaces of the manifold (for example, localizing some orbitals more than others). Applications to model systems and more realistic low-dimensional materials are presented.\\[4pt] [1] Marzari et al. Rev. Mod. Phys. 84, 1419 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 12:39PM - 12:51PM |
N24.00008: Improving the Accuracy of Diffusion Monte Carlo: Insights from Calculations of High Pressure Solid-Solid Phase Transitions L. Shulenburger, T. R. Mattsson A challenging application for any electronic structure method is the calculation of solid-solid phase transitions under pressure. Due to stringent requirements on accuracy imposed by the sensitivity of such transitions on small changes in free energy these calculations are exquisitely sensitive to any systematic errors in the calculations. In this talk we will review the present sources of methodological uncertainties in the diffusion quantum Monte Carlo (DMC) technique and study their effects on the calculation of solid-solid phase transitions. Particular attention will be paid to finite size effects and errors arising from the use of pseudopotentials. \\ \\ Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 20, 2013 12:51PM - 1:03PM |
N24.00009: Assessing the connection between charge density and local fields Eric Shirley The dielectric screening of a potential disturbance depends on the electronic charge density. Local-field effects, such as those which generate non-zero off-diagonal matrix elements of the dielectric matrix, are related to Fourier components of the charge density. This talk will review the degree to which one can predict such off-diagonal effects based on the charge density alone. This shall be done within an independent-particle approximation and using model dielectric functions so that the results can be compared. We shall sample a range of metals, semiconductors and insulators. [Preview Abstract] |
Wednesday, March 20, 2013 1:03PM - 1:15PM |
N24.00010: Anomalous Anharmonic Phonons in PbTe Reproduced from First-Principles Calculations Yue Chen, Chris Marianetti PbTe is of great interest due to its thermoelectric properties. Inelastic neutron scattering experiments reveal a signature of strong anharmonicity as evidenced in an anomalous temperature dependence of the phonon spectra. Novel approaches based on first-principles calculations have been developed for computing anharmonic phonons at elevated temperatures in recent years, though these techniques do not include lifetime effects and hence cannot address the anomalies observed in experiment. Here we perform first-principles molecular dynamics which includes the anharmonic terms at lowest order. The temperature dependent phonon spectra is computed and compared to experimental measurements, yielding insight on the origin of the observed anomalies. [Preview Abstract] |
Wednesday, March 20, 2013 1:15PM - 1:27PM |
N24.00011: Endohedral fullerene as acceptor: A DFT study on charge transfer states of Sc$_3$N@C$_{80}$-porphyrin complex Fatameh Amerikheirabadi, Luis Basurto, Rajendra Zope, Tunna Baruah C$_{60}$ fullerene and its derivatives are the most popular acceptors which are used in molecular/polymeric complexes used in organic photovoltaics. Endohedral fullerenes are shown to produce long lived charge separated states. The Sc$_3$N@C$_{80}$, the third most abundant fullerene after C$_{60}$ and C$_{70}$, has a larger cage with a radius of 4.1 Ang. We have carried out a DFT study on the electronic structure of ground and charge transfer states of a model Sc$_3$N@C$_{80}$-Zn tetraphenyl porphyrin cofacial complex. The C$_{80}$ cage used in our calculations has icosahedral symmetry. We find that the lowest charge transfer state with a hole on the porphyrin and an electron on the Sc$_3$N@C$_{80}$ is at 2.1 eV above the ground state. The calculations show that different orientations of the Sc$_3$N unit to the porphyrin plane do not significantly alter the electronic structure. The electronic structure of the complex and its components along with the exciton binding energies will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 1:27PM - 1:39PM |
N24.00012: ABSTRACT WITHDRAWN |
Wednesday, March 20, 2013 1:39PM - 1:51PM |
N24.00013: Melting of ice simulated by a multicanonical method combined with a first-principles calculation Yoshihide Yoshimoto Water is a ubiquitous material and is both scientifically and technologically important. For the simulation of water, the most common PBE semi-local exchange correlation (XC) functional has an issue: it gives over-structured liquid compared to the experimental one for a given temperature. On the other hand, the PBE0 hybrid XC functional was claimed to be better for the description of water recently [1,2]. In this study, the melting of ice, one of its most fundamental property, was simulated by a multicanonical method combined with a first-principles calculation [3,4]. Both the PBE XC functional and the PBE0 hybrid XC functional were adopted for the simulation. With accelerated computation of the hybrid functional by GPGPU, it was found that the PBE0 XC hybrid functional gave an improved melting temperature compared to that by PBE [5].\\[4pt] [1] C. Zhang and G. Galli et al., J. Chem. Theory and Comput., 7, 1443 (2011).\\[0pt] [2] B. Santra and M. Scheffler et al., J. Chem. Phys., 131, 124509 (2009).\\[0pt] [3] Y. Yoshimoto, J. Chem. Phys., 125, 184103 (2006).\\[0pt] [4] Y. Yoshimoto, J. Phys. Soc. Jpn., 79, 034602 (2010).\\[0pt] [5] S. Yoo, X.C. Zeng, and S.S. Xantheas, J. Chem. Phys., 130, 221102 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 1:51PM - 2:03PM |
N24.00014: DCA$^+$: Incorporating self-consistently a continuous momentum self-energy in the Dynamical Cluster Approximation Peter Staar, Thomas Maier, Thomas Schulthess The dynamical cluster approximation (DCA) is a systematic extension beyond the single site approximation of dynamical mean field theory (DMFT) to include spatially non-local correlations in quantum many-body simulations using a finite size embedded cluster. In the last decade, the DCA has been very useful in describing and analyzing phase transitions in models of correlated electron systems such as the single-band Hubbard model. In the standard DCA algorithm, the single-particle self-energy is approximated by a step function in momentum space, with constant values in regions about the cluster momenta. As a consequence, results often depend sensitively on the topology and morphology of the chosen cluster and the corresponding cluster momenta. Here, we present an extension to the standard DCA that incorporates a self-energy with smooth, continuous momentum dependence self-consistently in the DCA algorithm. In this new algorithm, the influence of the cluster-geometry is significantly reduced and the self-energy converges much more rapidly as a function of cluster-size. We demonstrate the improved convergence of this algorithm for results of the pseudo-gap temperature $T^*$ and the superconducting temperature $T_c$ versus cluster-size. [Preview Abstract] |
Wednesday, March 20, 2013 2:03PM - 2:15PM |
N24.00015: Polaron Localization in Conjugated Polymers by Hybrid DFT Methods Nan Shao, Qin Wu Reliable application of density functional theory (DFT) to study the electronic properties of polarons remains controversial. A proper description should exhibit both the formation of a charge-localized electronic state and saturation of the polaron size for increasing oligomer length. The aim of this work is to find a proper hybrid DFT method to study the chain length related electronic properties of charged conjugated polymer system. Using oligopyrrole cations as a test case, global hybrid functionals such as BHandHLYP can show charge localization, but a well-defined polaron size does not emerge when the length of the oligomer is increased; the saturation effect was not predicted correctly. By applying 100{\%} long-range corrected hybrid functionals, LRC-PBE, the saturation of charge distribution has been achieved, implying that the LRC-PBE is a better way to describe the spatial extent of the electronic state of polypyrrole than the conventional hybrid functionals. The tuning of the range parameter and the study of other polymer polaron systems will be discussed. [Preview Abstract] |
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