Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S18: Electronic Structure Methods IFocus

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Sponsoring Units: DCOMP Chair: Jianwei Sun, Temple University Room: BCEC 156B 
Thursday, March 7, 2019 11:15AM  11:27AM 
S18.00001: van der Waals density functional with corrected longrange behavior Timo Thonhauser, Kristian Berland, Debajit Chakraborty A new van der Waals density functional, i.e. vdWDF3, has been developed by maintaining an accurate balance between short and longrange contributions to the nonlocal correlation in the original vdWDF scheme. We stay entirely true to the original design criteria of vdWDF, adhering to the exact same physical constraints. But, we take advantage of some freedom in defining the plasmondispersion model ω_{q}, which allows us to significantly improve the asymptotic behavior and the resulting calculated C_{6} coefficients, overcoming the problem of unreliable C_{6} coefficients from previous vdWDF functionals. Our functional predicts very well the binding energies (<5% error) and separation for molecular dimers of commonly used reference datasets (such as S22, A24, X40, S66 etc.); performance in solids, layered structures, and noble gas dimers is also good. 
Thursday, March 7, 2019 11:27AM  11:39AM 
S18.00002: A fitted van der Waals density functional with accurate short and longrange interactions Debajit Chakraborty, Kristian Berland, Timo Thonhauser A fitting scheme has been developed to create a new van der Waals density functional, called vdWDFfit. The original vdWDF kernel, as well as the plasmon dispersion model, are numerically modified and fitted to minimize the binding energy and separation errors with respect to experimental/RPA/highlevel quantum chemistry results of a list of commonly used reference datasets (S22, A24, X40, S66 etc.). We are also pursuing automated fitting schemes using machinelearning techniques. This is work in progress and we are reporting first, promising results. 
Thursday, March 7, 2019 11:39AM  11:51AM 
S18.00003: A Generalized Gradient Approximation with Local Parameters. Angel Albavera Mata, Karla Botello Mancilla, Daniel MejiaRodriguez, Sam B Trickey, Jose L Gazquez, Alberto Vela The dependence of typical generalized gradient approximations (GGA) for exchangecorrelation (XC) on fixed, global parameters makes them incapable of providing good accuracy in predicting, simultaneously, molecular and solid properties. This drawback has motivated recent interest in constructing more flexible GGAs incorporating XC parameters that depend on the density, bounding their values to those for the high and lowdensity regimes. In this work we explore a scheme to couple X and C functionals through the densitydependence of the gradient expansion coefficient derived beyond the random phase approximation [1,2] employed in different C functionals [35]. We discuss the implications of such local dependence of the gradient coefficient and show results of these GGAs for various properties involving test sets of atoms, molecules, and solids. 
Thursday, March 7, 2019 11:51AM  12:03PM 
S18.00004: NeuralNetwork Implementation of Transferable KohnSham ExchangeCorrelation Functionals Ryo Nagai, Ryosuke Akashi, Shu Sasaki, Shinji Tsuneyuki, Osamu Sugino The accuracy of KohnSham density functional theory [1] depends crucially on its approximated exchangecorrelation functional. Structures of conventional density functionals depend on, however, combination of a number of physical constraints, making thereby the construction technically demanding and systematic improvement of their accuracy difficult. In order to overcome this difficulties, we present a new strategy free from the use of complicated physical constraints: representing the functional structure with Neuralnetwork (NN), which is an extremely flexible function with a large number of parameters. 
Thursday, March 7, 2019 12:03PM  12:15PM 
S18.00005: On the Physical Origin of the 1/4 Exact Exchange in Density Functional Theory Marco Bernardi This talk discusses the physical origin of the widely used 1/4 fraction of exact exchange in density functional theory (DFT)’s hybrid functionals. We show that this 1/4 fraction of exact exchange can be attributed to spinflip exchange interactions between pair states with antiparallel spin, which arise from the coupling between doublyexcited configurations contributing to the ground state energy. To obtain this result, we compare two models of the exchange energy of an electron pair in an unpolarized electron gas. One model treats the spin states of the electron pair as a statistical ensemble, while the other includes their quantum superposition. The quantum mechanical treatment is shown to possess an exchange energy equal to that of the statistical ensemble, plus an additional 1/4 exact exchange energy due to a spinflip exchange interaction between electron pair states with antiparallel spin. By showing that the statistical ensemble contains 3/4 of the densitybased exchange energy, we can attribute, for each pair of occupied orbitals, the 1/4 exact exchange used in hybrid DFT to the spinflip exchange processes mentioned above. Implications including the tendency of the LDA to overbind and the performance of hybrids for fewelectron systems will be discussed. 
Thursday, March 7, 2019 12:15PM  12:27PM 
S18.00006: The Quasi2D Electron Gas and Density Functional Theory: Finding a Finite Limit Aaron Kaplan, Kamal Wagle, John P Perdew The uniform electron gas in three and two dimensions is treated exactly by popular KohnSham density functional approximations. However, no generalpurpose semilocal functional can find the correct behavior of a 3D electron gas undergoing extreme compression in one dimension. In this talk, I will present our recent work [1] applying the SCAN functional to this perennial problem. While the exact exchangecorrelation energy per electron tends to a finite 2D limit, the local density and generalized gradient approximations to it diverge to minus infinity. SCAN tends to a finite limit that is however an order of magnitude too negative. These errors at high compression are in one sense harmless, since the noninteracting kinetic energy, treated exactly in KohnSham density functional theory, overwhelms them. Relevant background in KohnSham density functional theory will be presented, and only passing familiarity is assumed. 
Thursday, March 7, 2019 12:27PM  12:39PM 
S18.00007: Response in the local, nonnegative kinetic energy density of a perturbed free electron gas: potential functionals and density functionals William C Witt, Emily Ann Carter We discuss two sets of response functionals for estimating the local, nonnegative kinetic energy density of initially free electrons that are perturbed by a static external potential. We begin with first and secondorder functionals of the perturbing potential, which are directly analogous to known response functionals for the electron density and the integrated kinetic energy. We provide reciprocalspace formulations of these functionals that complement previously known realspace expressions. We then present alternate first and secondorder response functionals that operate on the induced electron density and do not depend explicitly on the applied potential. The structure of these latter functionals will help guide the design of the more sophisticated kinetic energy functionals that enable orbitalfree density functional theory simulations of materials. We examine the performance of the response functionals, in densityfunctional form, when applied to electron densities generated from local pseudopotential calculations for Li, Al, and Si solids. 
Thursday, March 7, 2019 12:39PM  12:51PM 
S18.00008: Large Z scaling of Atomic Pauli potentials Jeremy Redd, Antonio C Cancio Modeling the Pauli energy, the contribution to kinetic energy of Pauli statistics, without using orbitals is an open problem for electronic structure theory. One aspect of this problem is correctly producing the Pauli potential, which is its response to a change in density. A powerful tool to analyze any density functional quantity is LiebSimon scaling, taking its limit in nonrelativistic neutral atomic systems as nuclear charge and particle number approach infinity. We calculate the exact orbitaldependent Pauli potential for closedshell atoms out to element Z=976. Lieb and Simon proposed five distinct regions of behavior in the large Z limit, of which our results show four: a region of constant potential near the nucleus, a core where the potential oscillates about the TF potential, a transition region where the potential deviates unexpectedly from the TF potential, and an evanescent region where it decays exponentially. We compare the exact potential to several models that are variations on the gradient expansion to test their utility as orbitalfree approximations. This research may provide insight into semiclassical description of Pauli statistics, and new limiting behaviors to aid the improvement of orbitalfree DFT functionals. 
Thursday, March 7, 2019 12:51PM  1:03PM 
S18.00009: Koopmans compliance: a functional theory for spectral properties Andrea Ferretti, Nicola Colonna, Ngoc Linh Nguyen, Nicola Marzari Energy functionals which depend explicitly on each individual orbital density, 
Thursday, March 7, 2019 1:03PM  1:15PM 
S18.00010: The SelfConsistent Field in KohnSham Density Functional Theory: A Review of Methods and Algorithms Nick Woods, Michael C Payne, Phil Hasnip Software that computes the electronic structure of a material by searching for an infimum of the KohnSham energy functional  achieving selfconsistency  requires the use of an algorithm that iterates an initial estimate of the particle density toward the selfconsistent particle density. We present a review of methods and algorithms that solve this problem in the context of planewave, pseudopotential density functional theory. 
Thursday, March 7, 2019 1:15PM  1:27PM 
S18.00011: Obtaining Stationary States in Density Functional Theory Using Imaginary Time Propagation Cedric Flamant, Grigory Kolesov, Efstratios Manousakis, Efthimios Kaxiras Density functional theory (DFT) is widely successful for determining electronic and structural properties of materials and molecules. In order to perform a DFT calculation, the KohnSham nonlinear equations must be solved selfconsistently, a task usually achieved using selfconsistent field (SCF) loops. However, for large systems SCF can occasionally struggle to find the ground state. Using timedependent DFT in imaginary time, a given starting state can be purified into its lowestenergy component with reliable monotonic convergence. With appropriate constraints, it is also possible to obtain excited states with imaginary time propagation (ITP). We compare and analyze the performance of ITP and SCF in a few difficult systems, and present a few excited state calculations performed with the method. 
Thursday, March 7, 2019 1:27PM  1:39PM 
S18.00012: Pathintegral imaginarytime TDDFT simulation of the hydrogenbonded systems Grigory Kolesov, Efstratios Manousakis, Efthimios Kaxiras We have recently developed a new imaginarytime timedependent densityfunctional theory (itTDDFT)based approach for computing the groundstate electronionic structure of atomistic systems[1]. In this method the ionic degrees of freedom are treated in the pathintegral formalism, whereas electronic degrees of freedom are treated with itTDDFT. Our method, given an exact functional of the electronic density, is in principle exact in the zerotemperature limit. With approximate functionals it offers a practical path to direct ab initio simulation of molecular and condensed matter systems going beyond the BornOppenheimer approximation. In this work we apply our method to study properties of hydrogenbonded and tautomeric molecules: hydrogen sulfide, formic acid and terephthalic acid dimers. We compare the results of our simulations to known experimental and theoretical results. 
Thursday, March 7, 2019 1:39PM  1:51PM 
S18.00013: Benchmarking the structure selection performance of the SCAN functional relative to PBE and PBE+D3 Julia Yang, Daniil Kitchaev, Gerbrand Ceder The ability of firstprinciples computational methods to reproduce groundstate crystal structure is key to their application in the study of structural phase transitions and the discovery of new materials. In assessing the reliability of structure selection made on the basis of densityfunctional theory, it is critical to understand the impact of various errors on structure stabilization arising from the choice of density functional. Here, we evaluate the SCAN functional performance in structure selection relative to widelyused PBE and PBE+D3 functionals and build on recent reports that the SCAN functional significantly improves crystal structure selection across a wide range of main group compounds [1]. We demonstrate that the origin of this improvement is to a large extent the inclusion of van der Waals interactions. 
Thursday, March 7, 2019 1:51PM  2:03PM 
S18.00014: From one to three, exploring the rungs of Jacob’s ladder in magnetic alloys Matthieu Verstraete, Aldo H Romero Magnetic systems represent an important challenge for electronic structure methods, in particular Density Functional Theory (DFT). We benchmark different exchange correlation (xc) functionals with respect to each other, and with respect to available experimental data, on two families of binary iron alloys. We climb three rungs in Jacob’s ladder of DFT (i) the local density approximation, (ii) the industry standard approximations due to Perdew, Burke and Ernzerhof, and PBEsol, and finally (iii) the metaGGA functional SCAN. More than 400 structures in ferromagnetic and antiferromagnetic configurations were considered. We compare the Convex Hull, magnetic moment, structure, and formation energy. None of the functionals work in all conditions: the GGAs and mGGA give a fair crystal structure, but SCAN strongly overestimates the formation energy (wrt PBE and experiment). Magnetic moments are better predicted by PBE as well. Our results show that magnetic and strongly correlated materials are a tough litmus test for DFT, and that they represent the next frontier towards a truly universal xc functional. 
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