Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M24: Electronic Structure Methods I |
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Sponsoring Units: DCOMP Chair: Wenguang Zhu, University of Tennessee Room: 326 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M24.00001: Role of electronic localization in the phosphorescence of iridium sensitizing dyes Burak Himmetoglu, Alex Marchenko, Ismaila Dabo, Matteo Cococcioni In this talk we present a recent systematic study\footnote{B. Himmetoglu, A. Marchenko, I. Dabo and Matteo Cococcioni, J. Chem. Phys. {\bf 137}, 154309 (2012)} of three representative iridium dyes, namely, Ir(ppy)$_3$, FIrpic and PQIr, which are commonly used as sensitizers in organic optoelectronic devices. We show that electronic correlations play a crucial role in determining the excited state energies in these systems, due to localization of electrons on Ir $d$ orbitals in the ground state. Electronic localization is treated by employing hybrid functionals within time-dependent density functional theory (TDDFT) and with Hubbard model based corrections within the $\Delta$-SCF approach. The performance of both methods are studied in a comparative fashion and shown to be in good agreement with experiments (within a few tenths of an electron-volt in predicting singlet-triplet splittings and optical resonances). The Hubbard corrected functionals provide further insights on the charge-transfer character of the excited states. The gained insight allows us to comment on envisioned functionalization strategies to improve the performance of these systems. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M24.00002: A Strategy for Finding a Reliable Starting Point for G$_0$W$_0$ Demonstrated for Molecules Thomas Korzdorfer, Noa Marom Many-body perturbation theory in the G$_0$W$_0$ approximation is an increasingly popular tool for calculating electron removal energies and fundamental gaps for molecules and solids. However, the predictive power of G$_0$W$_0$ for molecules is limited by its sensitivity to the density functional theory (DFT) starting point. In this contribution, the starting point dependence of G$_0$W$_0$ is demonstrated for several small organic molecules. Analysis of the starting point dependence leads to the development of a non-empirical scheme that allows to find a consistent and reliable DFT starting point for G$_0$W$_0$ calculations by adapting the amount of Hartree-Fock-exchange in a hybrid DFT functional. The G$_0$W$_0$ spectra resulting from this {\it consistent starting point (CSP) scheme} reliably predict experimental photoelectron spectra over the full energy range. This is demonstrated for a test set of various typical organic semiconductor molecules.\\[4pt] [1] T. Korzdorfer and Noa Marom, Phys. Rev. B Rapid Communications {\bf 86}, 041110 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M24.00003: A Benchmark of GW Methods for Azabenzenes: Is the GW Approximation Good Enough? Noa Marom, Fabio Caruso, Xinguo Ren, Oliver Hofmann, Thomas K\"orzd\"orfer, James Chelikowsky, Angel Rubio, Matthias Scheffler, Patrick Rinke Many-body perturbation theory in the \textit{GW} approximation is a useful method for describing electronic properties associated with charged excitations. A hierarchy of \textit{GW} methods exists, starting from non-self-consistent $G_{\mathrm{0}}W_{\mathrm{0}}$, through partial self-consistency in the eigenvalues (ev-scGW) and in the Green's function (sc\textit{GW}$_{\mathrm{0}})$, to fully self-consistent GW (sc\textit{GW}). Here, we assess the performance of these methods for benzene, pyridine, and the diazines. The quasiparticle spectra are compared to photoemission spectroscopy (PES) experiments with respect to all measured particle removal energies and the ordering of the frontier orbitals. We find that the accuracy of the calculated spectra does not match the expectations based on their level of self-consistency. In particular, for certain starting points $G_{\mathrm{0}}W_{\mathrm{0}}$ and sc\textit{GW}$_{\mathrm{0}}$ provide spectra in better agreement with the PES than sc\textit{GW}. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M24.00004: Local atomic energies from optimal atomic orbitals Bj\"orn Lange, Christoph Freysoldt, J\"org Neugebauer Decomposing the energy of a condensed matter system into atomic contributions is of great use e.g. for understanding the physical origin of defect and surface energetics or for identifying chemically reactive regions in disordered systems. However, commonly employed energy calculations in the framework of density-functional theory (DFT) do not in general provide a natural decomposition into atoms. Here we propose a novel scheme to achieve this based on the recently introduced concept of atom-centered Quamols [1] that are variationally optimized to represent the electronic structure with a minimal basis set, which largely avoids local overcompleteness issues. The spillage resulting from the remaining small incompleteness is segmented according to a space separation derived from the Quamol atomic densities, maintaining the accuracy of the underlying DFT calculation. The total energy is then decomposed by combining this basis set with a local energy density treatment based on the ideas of Chetty and Martin [2]. We demonstrate the performance of our scheme by visualizing and analyzing the energy distribution at surfaces and in amorphous silicon.\\[4pt] [1] Lange, B et al., Phys. Rev. B 84, 085101, (2011)\\[0pt] [2] Chetty, N. and Martin, Richard M., Phys. Rev. B 45, 6074, (1992) [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M24.00005: Fast response function for finite and bulk systems Peter Koval, Federico Marchesin, Daniel Sanchez Portal, Dietrich Foerster Many-body perturbation theory of bulk systems is often realized within reciprocal space, using plane-wave (PW) basis sets. PW basis is advantageous because of its elementary basis functions and simple convergence control. However, the number of functions in PW basis grows with third power of unit cell size, irrespective of actual number of atoms present in the unit cell. Moreover, PW basis gives rise to full matrices in tensor algebra due to space-filling nature of PW. An alternative to PW would be usage of localized basis functions. In this contribution, we show how a basis of \textit{dominant products} (DP) can be used to describe excitations in finite and bulk systems. We present calculations of absorption spectra and electron-energy loss spectra within time-dependent density functional theory, realized within DP basis. The usage of localized functions and iterative techniques allow to keep the complexity of the calculations rather low: the overall number of operations grows with third power of number of atoms in the unit cell.Moreover, we have recently shown that Hedin's $GW$ calculations can also be performed using DP basis with an order-$N^3$ scaling for finite systems. We are currently extending this $GW$ methodology to bulk systems. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M24.00006: Density-Functional Theory Applied to Rare Earth Metals: Approaches Based on the Random-Phase Approximation Marco Casadei, Xinguo Ren, Patrick Rinke, Matthias Scheffler, Angel Rubio The description of the volume collapse exhibited by some \emph{rare earth} metals poses a great challenge to density-functional theory (DFT) since local/semilocal functionals (LDA/GGA) fail to produce the associated phase transitions. We approach this problem by treating all electrons at the same quantum mechanical level, using both hybrid functionals (e.g. PBE0 and HSE06) and exact-exchange plus correlation in the random-phase approximation (EX+cRPA). We also assess the performance of recently developed beyond RPA schemes (e.g. rPT2 [1]). The calculations are performed for cerium and praseodymium, that display a volume collapse, and neodymium, in which the volume collapse is absent. The isostructural $\alpha$-$\gamma$ phase transition in cerium is the most studied. The exact exchange contribution in PBE0 and HSE06 is crucial to produce two distinct solutions that can be associated with the $\alpha$ and $\gamma$ phases, but quantitative agreement with the extrapolated phase diagram requires EX+cRPA [2].\\[4pt] [1] Ren \emph{et al.}, J. Mater. Sci. \textbf{47}, 7447 (2012).\\[0pt] [2] M. Casadei {\it et al.}, Phys. Rev. Lett. \textbf{109}, 14642 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M24.00007: Vibrational spectroscopy of liquid water from first principles simulations: Raman Spectra Quan Wan, Leonardo Spanu, Giulia Galli, Francois Gygi Raman spectroscopy is an important probe of the structural and vibrational properties of aqueous solutions and of water at interfaces. While many experimental data are available for various systems, no results of ab initio computations have yet been reported for the Raman spectra of liquid water or solutions. We computed the Raman spectrum of water at ambient conditions using first principles molecular dynamics simulations, coupled to the calculation of polarizability within density functional perturbation theory. We used semi-local functionals, 64 molecule cells and the Qbox code. Our results are in satisfactory agreement with experiment. We provided an interpretation of the spectral features oberved at low frequency and within the stretching band by defining a polarizability of water molecules in the fluid. Coupling the calculation of Raman and IR spectra is in progress: such coupling will open the way to interpret advanced vibrational spectroscopy measurements, e.g. Sum Frequency Generation spectroscopy. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M24.00008: The bond-breaking and bond-making puzzle: many-body perturbation versus density-functional theory Fabio Caruso, Daniel Rohr, Maria Hellgren, Xinguo Ren, Patrick Rinke, Angel Rubio, Matthias Scheffler Diatomic molecules at dissociation provide a prototypical situation in which the ground-state cannot be described by a single Slater determinant. For the paradigmatic case of H$_2$-dissociation we compare state-of-the-art many-body perturbation theory in the $GW$ approximation and density-functional theory (DFT) in the exact-exchange plus random-phase approximation for the correlation energy (RPA). Results from the recently developed renormalized second-order perturbation theory (rPT2) are also reported. For an unbiased comparison and to prevent spurious starting point effects both RPA and $GW$ are iterated to {\it full} self-consistency (i.e. sc-RPA and sc-$GW$). Both include topologically identical diagrams for the exchange and correlation energy but are evaluated with a non-interacting Kohn-Sham and an interacting $GW$ Green function, respectively. This has profound consequences for the kinetic and the correlation energy. $GW$ and rPT2 are both accurate at equilibrium, but fail at dissociation, in contrast to sc-RPA. This failure demonstrates the need of including higher order correlation diagrams in sc-$GW$. Our results indicate that RPA-based DFT is a strong contender for a universally applicable electronic-structure theory. F. Caruso {\it et al.} arxiv.org/abs/1210.8300. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M24.00009: Structural and Electronic Properties of the Solvated Chloride Ion from First Principles Simulations Francois Gygi, Cui Zhang, Tuan Anh Pham, Giulia Galli First principles simulations of anions in aqueous solutions represent a challenging task both from a theoretical and computational standpoint, and only sporadic ab initio studies of their electronic properties have appeared in the literature. We carried out first principles molecular dynamics (MD) simulations of the chloride anion in liquid water with semi-local (PBE) and hybrid (PBE0) functionals, using the Qbox code. We found substantial differences in the orientation of the water molecules in the first anion solvation shell when using the two different levels of theory. Most importantly, the relative energies of the highest occupied level (HOMO) of the anion was found to be lower than the top of the valence band of water with PBE and the HOMO state is fairly delocalized, while it is higher with PBE0 and the corresponding state is localized on the anion. Although qualitative correct, the result obtained with PBE0 is only in fair agreement with experiment. It is only when using many body perturbation theory at the GW level and PBE0 trajectories that we could find qualitative and quantitative agreement with experiment [1]. Work supported by DOE-CMSN DE-SC0005180 and DOE-BES DE-SC0008938.\\[4pt] [1] P. Delahay, Acc. Chem. Res. 15, 40 (1982). [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:00AM |
M24.00010: Ab initio calculations of quasiparticle energies of solids, liquids and molecules using a spectral decomposition of the dielectric matrix Tuan Anh Pham, Huy-Viet Nguyen, Dario Rocca, Giulia Galli We recently developed a method for the calculation of quasiparticle energies within many body perturbation theory, at the $GW$ level, which avoids costly summations over empty electronic states and does not require the use of plasmon-pole models [1]. We present a comprehensive validation of this method, encompassing calculations of i) the vertical ionization energies of a set of over 80 molecules (containing from 14 to 424 valence electrons); ii) the relative position of energy levels of anions and water in hydrated sulfate and chloride clusters; iii) the band structure of a variety of semiconductors and (iv) the electronic properties of amorphous and liquid systems. The efficiency of our approach allowed us to compute quasiparticle energies of multiple configurations of liquid water, using samples with 64 molecules, selected over trajectories generated by ab initio molecular dynamics simulations. \\[4pt] [1] H. Viet Nguyen, T. Anh Pham, D. Rocca and G. Galli, Phys. Rev. B \textbf{85}, 081101(R) (2012); T. Anh Pham, H. Viet Nguyen, D. Rocca and G. Galli (submitted) [Preview Abstract] |
Wednesday, March 20, 2013 10:00AM - 10:12AM |
M24.00011: Structural Stability Driven by the Spin-Orbit Coupling and the Superconductivity in simple-cubic Polonium Chang-Jong Kang, Kyoo Kim, B.I. Min Polonium is the only element which has the simple-cubic (SC) structure in the periodic table. We have studied its structural stability based on the phonon dispersion calculations using the first-principles all-electron full-potential band method. We have demonstrated that the strong spin-orbit coupling (SOC) in SC-Po suppresses the Peierls instability and makes the SC structure stable. We have also discussed the structural chirality realized in beta-Po, as a consequence of the phonon instability. Further, we have investigated the possible superconductivity in SC-Po, and predicted that it becomes a superconductor with Tc $\sim$ 4 K at ambient pressure. The transverse soft phonon mode at q $\sim$ 2/3 R, which is greatly affected by the SOC, plays an important role both in the structural stability and the superconductivity in SC-Po. We have explored effects of the SOC and the volume variation on the phonon dispersions and superconducting properties of SC-Po. [Preview Abstract] |
Wednesday, March 20, 2013 10:12AM - 10:24AM |
M24.00012: Parallelized electronic transport calculations in real space Baruch Feldman, Oded Hod, Tamar Seideman, Leeor Kronik We present a real-space method for first-principles nano-scale electronic transport calculations, using the non-equilibrium Green's function (NEGF) method and complex absorbing potentials (CAPs) to represent the effects of the semi-infinite leads. In real space, the electronic Hamiltonian from Density Functional Theory (DFT) is very sparse. As a result, the transport problem parallelizes naturally and can scale favorably with system size. We illustrate our method with calculations on several realistic test systems and find good agreement with a reference calculation. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M24.00013: Surface chemistry in a full-potential QM/MM approach: making hybrids affordable Daniel Berger, Volker Blum, Karsten Reuter Nanostructured oxide surfaces are promising candidates for a wide range of energy and catalysis applications. When addressing corresponding functionalities through quantitative first-principles calculations, exploitation of the localized character of the chemical processes yields numerically most efficient approaches. To this end we augment the FHI-aims\footnote{V. Blum \textit{et al.}, Comput. Phys. Commun., \textbf{180}, 2175-2196 (2009)} package with a QM/MM\footnote{N. Bernstein \textit{et al.}, Rep. Prog. Phys., \textbf{72}, 026501 (2009)} functionality, in which the nanostructure and immediate oxide surrounding is described quantum mechanically, the long-range electrostatic interactions with the support are accounted for through a polarizable monopole field, and a shell of norm-conserving pseudopotentials correctly connects the two regions. We illustrate the accuracy and efficiency of the implementation with examples from the photo-catalytic water splitting context and specifically discuss the use of charged system states to address charge transfer processes. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M24.00014: Spin-Orbit Coupling within the GW Approximation Brad Barker, Jack Deslippe, Manish Jain, Johannes Lischner, Oleg Yazyev, Steven G. Louie We have developed and implemented an approach in which the effects of spin-orbit interactions to the quasiparticle band structure are incorporated within the GW approach, employing spinor wavefunctions computed at the density functional theory (DFT) level with fully relativistic pseudopotentials. Special consideration is given to the significance of the spin-dependent exchange-correlation potential. We compare these results to separate calculations where spin-orbit coupling is applied as a perturbation. We apply these methods to the properties of materials with heavy ion cores to determine the possible differences from the different treatments of spin-orbit coupling. [Preview Abstract] |
Wednesday, March 20, 2013 10:48AM - 11:00AM |
M24.00015: Cumulant expansion treatment of phonon contributions to the electron spectral function S.M. Story, J.J. Kas, M.J. Verstraete, J.J. Rehr We present an approach for calculations of phonon contributions to the electron spectral function at finite temerature based on cumulant expansion techniques. Our approach is based on a many--pole representation of the Eliashberg function for the electron--phonon interaction, calculations of the dynamical matrix using ABINIT [1], and an Einstein self--energy model [2]. The code has been implemented as part of a plug--in to ABINIT for calculations of various phonon properties, and is applicable to complex structures with several atoms per unit cell. Results are given for a number of systems and compared to those obtained with the GW approximation.\\[4pt] [1] X. Gonze et al., Computational Materials Science \textbf{25}, 478 (2002). \\[0pt] [2] A. Eiguren and C. Ambrosch--Draxl, Phys. Rev. Lett. \textbf{101}, 036402 (2008). [Preview Abstract] |
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