Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C02: Developments of DFT from Quantum to Statistical Mechanics (II)Focus

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Sponsoring Units: DCP DCOMP Chair: Weitao Yang, Duke Univ Room: LACC 150B 
Monday, March 5, 2018 2:30PM  3:06PM 
C02.00001: Finite Temperature Phase Diagrams by Nested Sampling Invited Speaker: Robert Baldock , Livia BartokPartay , Albert BartokPartay , Gabor Csanyi , Michael Payne The partition function describes the statistical properties of a system in thermodynamic equilibrium and most of the aggregate thermodynamic variables of the system, such as the total energy, free energy, entropy, and pressure, can be expressed in terms of the partition function or its derivatives. Knowledge of the partition function, thus, gives us access to complete finite temperature phase diagram of any material. However, no one has previously been able to compute the partition function for any atomistic system with realistic models of the interatomic interactions and, indeed, it has generally been believed that such a computation is intractable. However, the introduction of the ‘Nested Sampling’ technique has changed this situation. Nested Sampling is a novel inference algorithm developed by Prof John Skilling [1] that provides a method for searching the whole of configuration space in polynomial computational time – in contrast to methods such a simulated annealing for which there is no such rigorous mathematical bound on the amount of computational time needed. Interestingly, the Nested Sampling algorithm naturally samples configuration space in a way that is closely aligned with the contribution of each region of space to the partition function – high energy regions are sparsely sampled but low energy regions are more densely sampled. In this talk I shall describe how Nested Sampling can be used to calculate partition functions and I will describe a number of applications of the methodology. 
Monday, March 5, 2018 3:06PM  3:18PM 
C02.00002: Rocksalt or cesium chloride? Investigating the relative stability of the cesium halide structures with Random Phase Approximation based methods Niraj Nepal , Jefferson Bates , Adrienn Ruzsinszky The Random Phase Approximation (RPA) is gaining immense popularity as the benchmark of semilocal methods such as LDAs and GGAs, as it naturally incorporates weak interaction and eliminates selfinteraction error in its exchange component. By taking cesium halides as benchmarks for ionic solids, we present the comparative assessment of RPA based methods with the newly developed SCAN metaGGA, which incorporates the intermediaterange of dispersion [1]. We also assess various longrange dispersion corrections to SCAN such as SCAN+D3 and SCAN+rVV10. We have demonstrated that RPA is quite successful in describing the ground state properties of these alkali halides, especially smaller fluorides and chlorides. Beyond RPA methods such as rALDA with an exchangecorrelation kernel systematically improve upon RPA results for larger bromides and iodides. Though rALDA and rAPBE share a common formalism, rAPBE fails to predict the equilibrium properties of these halides. We have also demonstrated that the SCAN and dispersioncorrected SCAN can also accurately describe the equilibrium properties of these halides and can be good alternatives where computational power can be saved. 
Monday, March 5, 2018 3:18PM  3:30PM 
C02.00003: Understanding the role of image charges in mean field theories of ionic solutions. Francisco Solis Mean field theories for charged systems such as PoissonBoltzmann are of great practical importance. Standard applications include the description of molecular and colloidal interactions in an aqueous environment where ions are present. Recent simulations with asymmetric ionic mixtures have shown the existence of nontrivial behavior of ions near interfaces with dielectric contrast. Near these interfaces ion concentrations are increased or depleted in ways not predicted by the PoissonBoltzmann approach. The origin of this discrepancy can be traced to the omission, in the construction of the mean field theory, of the role of image charges. When these effects are included, a mean field theory can be formulated that addresses the observed phenomena. In this approach, the PoissonBoltzmann equation acquires an extra term describing the presence of interfaces by means of an effective onebody short range potential. 
Monday, March 5, 2018 3:30PM  3:42PM 
C02.00004: Deorbitalization of metaGGA exchange correlation functionals Daniel MejiaRodriguez , Sam Trickey

Monday, March 5, 2018 3:42PM  3:54PM 
C02.00005: The generalpurpose SCAN metaGGA for ferroelectric materials Yubo Zhang , Jianwei Sun , John Perdew , Xifan Wu The strongly constrained and appropriately normed (SCAN) metaGGA was found to be accurate for geometries and energies of diverselybonded materials. Here, we investigate its performance for the structural and electric properties of ferroelectric materials [1]. Ferroelectricity originates from the breaking of spatial inversion symmetry, and spontaneous polarization is determined by the polar structural distortion. Although DFT has been widely used for studying ferroelectric properties, an accurate prediction of the distortion is rather challenging and is limited by the adopted exchangecorrelation functionals. LDA is usually more reliable than the GGAs for perovskites, but LDA usually strongly underestimate the cell volumes. The B1WC hybrid functional was specially designed for ferroelectrics and systematically improves the calculated ferroelectric properties, however is computationally very expensive. Our results show that SCAN is comparable or more accurate than B1WC but with much cheaper computational cost. 
Monday, March 5, 2018 3:54PM  4:30PM 
C02.00006: Liquid drops on surfaces: using density functional theory to calculate the binding potential and drop profiles and comparing with results from mesoscopic modelling Invited Speaker: Andrew Archer For a film of liquid on a solid surface, the binding potential g(h) gives the free energy as a function of the film thickness h and also the closely related (structural) disjoining pressure Π = −∂g/∂h. The wetting behaviour of the liquid is encoded in the binding potential and the equilibrium film thickness corresponds to the value at the minimum of g(h). Several different methods based on density functional theory (DFT) for calculating g(h) are described. The first is the method we developed in the work of Hughes et al. [J. Chem. Phys. 142, 074702 (2015), 146, 064705 (2017)], that selfconsistently calculates an effective fictitious external potential that stabilises films with nonequilibrium value of h. A second method is to calculating g(h) using a Nudged Elastic Band (NEB) approach [Buller et al., J. Chem. Phys. 147, 024701 (2017)]. A third approach is to use an overdamped nonconserved pseudodynamics that finds g(h) as the trajectory through the free energy landscape. We also show that all three methods generate identical results for g(h). We illustrate these methods by presenting results from a simple discrete latticegas model and also for a LennardJones fluid and other simple liquids. The DFT used is based on fundamental measure theory and so incorporates the influence of the layered packing of molecules at the surface and the corresponding oscillatory density profile. The binding potential is frequently input in mesoscale models from which liquid drop shapes and even dynamics can be calculated. Here we show that the equilibrium droplet profiles calculated using the mesoscale theory are in good agreement with the profiles calculated directly from the microscopic DFT. For liquids composed of particles where the range of the attraction is much less than the diameter of the particles, we find that at low temperatures g(h) decays in an oscillatory fashion with increasing h, leading to highly structured terraced liquid droplets. 
Monday, March 5, 2018 4:30PM  4:42PM 
C02.00007: Accurate Parametrization of the NonInteracting Kinetic Energy of the Thermal Uniform Gas Kevin Yang , Kieron Burke There is currently tremendous interest in the XC properties of a thermal uniform gas.Here we construct an extremely accurate parametrization of the noninteracting kinetic energy for use in thermal density functional studies. Older parameterizations are either relatively crude or violate a simple relation involving the chemical potential. We use machinelearning (kernel ridge regression) to interpolate between the high and low temperature limits. 
Monday, March 5, 2018 4:42PM  4:54PM 
C02.00008: Trivial Crossing Problem in FSSH: dissection of different methods on a weakly coupled spirobifluorene Parmeet Nijjar , Tammie Nelson , Sebastian Fernandez Alberti , Sergei Tretiak , Oleg Prezhdo Tully’s fewest switches surface hopping (FSSH) has been a popular method for simulating quantumclassical dynamics in a large variety of systems. Its popularity has also brought forward its limitations. Trivial unavoided crossings between noninteracting states that may occur in complex molecular systems present a huge numerical challenge in FSSH. At such crossings, the nonadiabatic (NA) coupling is infinite at the exact crossing point and vanishingly small elsewhere. Finite timestep numerical propagation is likely to miss such crossings and give erroneous results like unphysical longrange energy transfer. The correct treatment of trivial crossings is contingent on their proper identification. Here, we consider the performance of two different strategies developed to deal with the trivial crossing problem in FSSH. Specifically, (i) the MinCost adiabatic state identity tracking algorithm, and (ii) the selfconsistent fewest switches surface hopping (SCFSSH) approach have been implemented within the Nonadiabatic Excited State Molecular Dynamics (NAESMD) framework. The performance of the methods has been investigated for the internal conversion and energy transfer dynamics of a weakly coupled spirolinked polyfluorene (spirobifluorene) system. 
Monday, March 5, 2018 4:54PM  5:06PM 
C02.00009: NonAdiabatic Dynamics in SingleElectron Tunneling Devices with TimeDependent Density Functional Theory Niklas Dittmann , Janine Splettstoesser , Nicole Helbig The recent advance of various singleelectron sources in solidstate setups has sparked interest in the investigation of electronic transport at the singleparticle level. In our recent work (N. Dittmann, J. Splettstoesser, N. Helbig, arxiv:1706.04547), we put forward timedependent densityfunctional theory to calculate the dynamics of interacting electrons in singleelectron tunneling devices. As a physical system, we analyze a singleelectron source which is built by a quantum dot tunnelcoupled to a nearby electron reservoir and driven by a timedependent gate voltage. By using analogies with quantumtransport theory, we extract a timenonlocal exchangecorrelation potential for a Hubbard U onsite interaction on the quantum dot. The time nonlocality manifests itself in a dynamical potential step, which we explicitly link to physical relaxation time scales of the electron dynamics. Finally, we discuss prospects for simulations of larger mesoscopic systems. 
Monday, March 5, 2018 5:06PM  5:18PM 
C02.00010: Reliable prediction of the reaction barrier height with HFDFT Suhwan Song , Eunji Sim , Kieron Burke Many systematic errors of semilocal DFT (GGA's and hybrids) are due to errors in the selfconsistent density, and so can be reduced by using a more accurate density. For anions, radicals in solution, stretched bonds, reaction barrier heights, and even spincrossover complexes, DFT results are substantially improved by using HartreeFock densities (called HFDFT). We revisit the case of reaction barriers, showing that while HFDFT can reduce barrier height errors by a factor of 2 or 3, even for hybrid functionals, different classes of barriers behave in different ways. 
Monday, March 5, 2018 5:18PM  5:30PM 
C02.00011: Why Do Hybrid Density Functionals and MetaGGAs Improve the Band Gaps of Solids in Generalized KohnSham Theory? John Perdew The fundamental gap of a solid is not only an excitation energy but also a groundstate second energy difference (ionization energy minus electron affinity). For an approximate functional constructed from a noninteracting density matrix, the gap between the occupied and unoccupied oneelectron energies, computed within a generalized KohnSham (GKS) scheme in which the energyminimizing exchangecorrelation potential is continuous and not constrained to be a multiplication operator, has been shown [1] to equal the groundstate second energy difference within the same approximation, so long as the created electron and hole delocalize fully over one, two, or three dimensions. Thus improvements of the groundstate second energy difference from local density and generalized gradient approximations (GGAs) to metaGGAs and then to hybrids explain the correspondingly improved bandstructure gaps, which are found only in GKS and not [2] in KS theory. 
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