Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M24: Electronic Structure Methods III |
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Sponsoring Units: DCOMP FIAP Chair: Alan Aspuru-Guzik, Harvard University Room: 203AB |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M24.00001: A Novel Gaussian-Sinc mixed Basis Set for Electronic Structure calculations Jonathan Jerke, Young Lee, C.J. Tymczak A Gaussian-Sinc mixed basis set for the computation of the electronic structure of atoms and molecules is presented. Excellent bases functions are known for ``core'' and ``valence'' separately, such as Gaussians for the ``core'' wave functions and Plane-waves for ``valance'' wave functions, but as yet no method is known that can accurately deal with both regimes in a single basis. A Gaussian-Sinc mixed basis can do both. This method resolves several issues such as: i) the Sincs basis spans the same space as the plane-waves basis, yet are semi-local enough to define all interaction elements including Exchange; ii) the Gaussians span the spherically symmetric core states and can be mixed with the Sinc functions in a computationally efficient methodology; iii) together, this mixed basis set is a flexible, computationally efficient and a highly accurate method for solving atomic and molecular problems. This methodology has been implemented within the Hartree-Fock level of theory within ultra-strong magnetic fields. To demonstrate the utility of this new method, we calculated the ground state Hartree-Fock energies to five digits accuracy in ultra strong magnetic fields for Helium to Neon, Molecular Hydrogen, Water, Carbon dioxide and Benzene. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M24.00002: Modeling the kinetic energy density of molecules -- towards an orbital-free meta-GGA Antonio Cancio, Dane Stewart, Jeremy Redd Driven by applications at high temperature and large system size, interest has recently turned to the construction of orbital-free density functionals, modeling the Kohn-Sham kinetic energy solely as a functional of the electron density and its derivatives. We report work on a metaGGA level orbital-free kinetic energy functional parametrized in terms of the local Laplacian and gradient of the density, and based on insights gained in the visualization of the kinetic energy density (KED) for atoms and the AE6 test set of molecules. Visualization of the KED, particularly in the region of localized electron pairs, such as atomic lone pairs and covalent bonds, pointed out significant flaws in an earlier metaGGA proposed by Perdew and Constantin. We find that these can be substantially fixed by revising an implicit constraint built into the prior model in the limit of strong electron localization -- when the Kohn-Sham KED approaches the bosonic limit. A first attempt at an improved model dramatically improves atomization energies for the AE6 test set, but remains an order of magnitude worse than a conventional Kohn-Sham GGA. We are currently working to fix a notable over-correction of the PC meta-GGA for the electron localization limit that may lead to further improvement. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M24.00003: Convergence of the phonon energy in two-dimensional atomic crystal of lead Jia-An Yan Accurate phonon energies are important for the study of two-dimensional (2D) atomic crystals. Using the 2D honeycomb lattice of lead (Pb) as a model system, we studied the convergence of the phonon energies on several important parameters in supercell calculations based on the density-functional perturbation theory as implemented in Quantum Espresso code. These parameters include the plane wave cut-off energy, the vacuum space size, the charge density cut-off, and FFT grid. The tested pseudopotentials (PPs) include the widely used Troullier-Martin (TM), Hartwigsen-Goedeker-Hutter (HGH), Projector Augmented-Wave (PAW), and ultrasoft pseudopotential (USPP), with the same PBE exchange-correlation functional. Surprisingly, the phonon energies calculated using these PPs exhibit quite distinct dependence on those parameters. Specifically, for both TM and USPP PPs, the phonon energies at the Brillouin zone center exhibit oscillations and even large negative phonon modes with the increase of the vacuum size. In contrast, the HGH and PAW PPs show fast and stable convergence with the same settings. The origin of these oscillation will be discussed. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M24.00004: Implicit solvent models in VASP Kiran Mathew, Richard Hennig Solid-liquid interfaces are at the heart of many modern-day technologies and presents challenge for materials simulation methods. A realistic first-principles computational study of such systems entails the inclusion of solvent effects. In our previous work, employing a linear implicit solvent model, we have demonstrated the importance of the inclusion of solvent effects on the calucaltions of reaction energy barriers and surface enegies of semiconductor nanocrystals. In this work we propose to extend the implicit solvent model to incorporate the effects of the ions in the solvent and also to include the effects of dilectric saturation phenomenon. A solvation model that includes the effects of ionic solution at a first principle level, takes us one step closer to a more realistic simulation of an electrochemical interface. Incorporating the dielectric saturation effects futher advance the capabilities of the state of the art DFT tools to study the Solid Electrolyte Interface(SEI) films formed on highly ionic surfaces such as Lithium halides. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M24.00005: Compact wavefunctions from compressed imaginary time evolution Jarrod McClean, Alan Aspuru-Guzik Simulation of quantum systems promises to deliver physical and chemical predictions for the frontiers of technology. The success of approximation methods for quantum systems has hinged on the relative simplicity of physical systems with respect to the exponentially complex worst case. Exploiting this relative simplicity has required detailed knowledge of the physical system under study. In this talk, I will introduce a general and efficient black box method for many-body quantum systems that utilizes technology from compressed sensing to find the most compact wavefunction possible without detailed knowledge of the system. It is a Multicomponent Adaptive Greedy Iterative Compression (MAGIC) scheme. No knowledge is assumed in the structure of the problem other than correct particle statistics. This method can be applied to many quantum systems such as spins, qubits, oscillators, or electronic systems. I will show the relation of this approach to matrix product states and discuss the implications. As a practical application, I use this technique to compute the ground state electronic wavefunction of hydrogen fluoride and recover $98$% of the basis set correlation energy or equivalently $99.996$% of the total energy with $50$ configurations out of a possible $10^7$. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M24.00006: First-Principles Studies of the Excited States of Chromophore Monomers and Dimers Samia Hamed, Sahar Sharifzadeh, Jeffrey Neaton Elucidation of the energy transfer mechanism in natural photosynthetic systems remains an exciting challenge. Through the careful analysis of excited states on individual chromophores and dimers -- and the predictive first-principles methods used to compute them -- we are building towards an understanding of the nature of excitation transfer among arrays of chromophores embedded in protein environments. Excitation energies, transition dipoles, and natural transition orbitals for the important low-lying singlet and triplet states of experimentally-relevant chromophores are obtained from first-principles time-dependent density functional theory (TDDFT) and many body perturbation theory. The effect of the Tamm-Dancoff approximation and the performance of several exchange-correlation functionals, including an optimally-tuned range-separated hybrid, are evaluated with TDDFT, and compared to MBPT calculations and experiments. This work has been supported by the DOE; computational resources have been provided by NERSC. [Preview Abstract] |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M24.00007: ``Phantom'' Modes in \textit{Ab Initio} Tunneling Calculations: Implications for Theoretical Materials Optimization, Tunneling, and Transport Sergey V. Barabash, Dipankar Pramanik Development of low-leakage dielectrics for semiconductor industry, together with many other areas of academic and industrial research, increasingly rely upon \textit{ab initio} tunneling and transport calculations. Complex band structure (CBS) is a powerful formalism to establish the nature of tunneling modes, providing both a deeper understanding and a guided optimization of materials, with practical applications ranging from screening candidate dielectrics for lowest ``ultimate leakage'' to identifying charge-neutrality levels and Fermi level pinning. We demonstrate that CBS is prone to a particular type of spurious ``phantom'' solution, previously deemed true but irrelevant because of a very fast decay. We demonstrate that (i) in complex materials, phantom modes may exhibit very slow decay (appearing as leading tunneling terms implying qualitative and huge quantitative errors), (ii) the phantom modes are spurious, (iii) unlike the pseudopotential ``ghost'' states, phantoms are an apparently unavoidable artifact of large numerical basis sets, (iv) a presumed increase in computational accuracy increases the number of phantoms, effectively corrupting the CBS results despite the higher accuracy achieved in resolving the true CBS modes and the real band structure, and (v) the phantom modes cannot be easily separated from the true CBS modes. We discuss implications for direct transport calculations. The strategy for dealing with the phantom states is discussed in the context of optimizing high-quality high-$\kappa $ dielectric materials for decreased tunneling leakage. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M24.00008: Three- to two-dimensional crossover in time-dependent density-functional theory Shahrzad Karimi, Carsten Ullrich Quasi-2D systems, such as an electron gas confined in a quantum well, are important model systems for many-body theories. Earlier studies of the crossover from 3D to 2D in ground-state DFT showed that local and semilocal exchange-correlation functionals which are based on the 3D electron gas are appropriate for wide quantum wells, but eventually break down as the 2D limit is approached. We now consider the dynamical case and study the performance of various linear-response exchange kernels in TDDFT. We compare approximate local, semilocal and orbital-dependent exchange kernels, and analyze their performance for inter- and intrasubband plasmons as the quantum wells approach the 2D limit. 3D (semi)local exchange functionals are found to fail for quantum well widths comparable to the 2D Wigner-Seitz radius, which implies in practice that 3D local exchange remains valid in the quasi-2D dynamical regime for typical quantum well parameters, except for very low densities. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M24.00009: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M24.00010: Ultrafast coupled plasmon-phonon mode dynamics in GaAs, a combined experimental and theoretical approach Evan Thatcher, Christopher Stanton, Kunie Ishioka, Amlan Basak, Hrvoje Petek We present results from a joint experimental and theoretical study exploring the excitation of coupled plasmon-phonon modes in GaAs. In contrast to previous coherent phonon studies in GaAs where electrons were generated primarily in the $\Gamma$ valley ($E_0$ gap), we use a pump-probe technique with a 10 fs pulse width and a shorter 400 nm laser wavelength to photoexcite electrons predominately in the L valley ($E_1$ gap). As a result: i) damping of the electron-hole plasma is faster and ii) diffusion of the carriers from the surface becomes important owing to the shorter absorption length. The probe pulses measure the time-dependent changes to the reflectivity due to the coupled plasmon-phonon modes created by the ultrafast photoexcitation and the subsequent depletion field screening. To model this, we solve for the time and density dependent coupled-mode frequencies allowing for ambipolar diffusion. Simulation of the coupled plasmon-phonon dynamics allows for comparison with, and a better understanding of experiments. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M24.00011: Surface Dangling Bonds Are a Cause of Type-II Blinking in Si Nanoparticles Nicholas Brawand, Marton Voros, Giulia Galli Exponential blinking statistics was reported in oxidized Si nanoparticles and the switching mechanism was attributed to the activation and deactivation of unidentified nonradiative recombination centers. Using ab initio calculations we predicted that Si dangling bonds at the surface of oxidized nanoparticles introduce defect states which, depending on their charge and local stress conditions, may give rise to ON and OFF states responsible for exponential blinking statistics. Our results are based on first principles calculations of charge transition levels, single particle energies, and radiative and nonradiative lifetimes of dangling bond defects at the surface of oxidized silicon nanoparticles under stress. [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M24.00012: Auxiliary density functionals: a new class of methods for efficient, stable density functional theory calculations Phil Hasnip, Matt Probert {\em Ab initio} materials modelling methods have become an essential tool for physical scientists in a wide variety of fields. The advent of more and more powerful computers has allowed larger, more complex systems to be simulated and the dramatic improvements in both experimental growth and characterisation methods have allowed the length scale of theoretical simulations and experimental studies to coincide at the nanoscale. Whilst there has been undoubted success in the modelling of nanomaterials, the approach is not without its problems. As the size of the simulation system is increased, the conventional algorithms used to find the electronic ground state often show poor convergence, and for large or complex systems they may fail to converge at all. We present a new class of methods for solving the Kohn-Sham equations based on constructing a mapping dynamically between the Kohn-Sham system and an auxiliary system. This auxiliary system is not required to be fermionic, and an exemplar bosonic scheme is presented which captures the key features of the Kohn-Sham behaviour. This auxiliary scheme is shown to provide good performance for a variety of bulk materials, and a substantial improvement in the scaling of calculations with system size for a range of materials. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M24.00013: Speeding up DFT: A new approach to k-point integration Jeremy J. Jorgensen, Derek C. Thomas, Matthew M. Burbidge, Ian H. Sloan, Conrad W. Rosenbrock, Rodney W. Forcade, Bret C. Hess, Gus L.W. Hart The bottleneck for high throughput material prediction is computational speed. Increasing the convergence rate of the band energy integration will decrease computation time. Band energy, despite its small contribution to the total energy, plays a large role in the calculation of formation enthalpies, where energy differences and not magnitudes are of greater importance. Current DFT codes generally choose k-points using the Monkhorst-Pack scheme, and then integrate the energy bands using the rectangle method. Instead, we interpolate the energy bands with splines, create a spline representation of the Fermi surface, and analytically integrate the energy bands beneath the Fermi surface to find the band energy. Our conservative estimate is a tenfold increase in computational efficiency for the band energy calculation. [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M24.00014: Universal continuum solvation models for plane-wave density functional theory Deniz Gunceler, T.A. Arias Electron density based continuum solvation models have progressed rapidly over the last few years. This has enabled the application of electronic structure methods, especially density functional theory, to many problems at solid-liquid interfaces. One limitation of these iso-density methods is that they are often parametrized only for aqueous solutions and not for many other solvents used in technological applications (for example, organic solvents). To overcome this defficiency, we present an algorithm for constructing ``universal'' isodensity models by using readily available thermodynamic quantities of the solvent (such as dielectric constant and vapor pressure). We also disscuss the applications of the resulting model, implemented in the open-source plane-wave code JDFTx, to the stability of anode surfaces in Lithium batteries. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M24.00015: Ab-initio atomic level stress and role of d-orbitals in CuZr, CuZn and CuY Madhusudan Ojha, Don M. Nicholson, Takeshi Egami Atomic level stress offers a new tool to characterize materials within the local approximation to density functional theory (DFT). Ab-initio atomic level stresses in B2 structures of CuZr, CuZn and CuY are calculated and results are explained on the basis of d-orbital contributions to Density of States (DOS). The overlap of d-orbital DOS plays an important role in the relative magnitude of atomic level stresses in these structures. The trends in atomic level stresses that we observed in these simple B2 structures are also seen in complex structures such as liquids, glasses and solid solutions. The stresses are however modified by the different coordination and relaxed separation distances in these complex structures. We used the Locally Self-Consistent Multiple Scattering (LSMS) code and Vienna Ab-initio Simulation Package (VASP) for ab-initio calculations. [Preview Abstract] |
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