Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K02: General Theory, Computational Physics |
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Sponsoring Units: DCOMP Chair: Raffaele Resta, Univ of Trieste - Trieste Room: LACC 150B |
Wednesday, March 7, 2018 8:00AM - 8:12AM |
K02.00001: Finding 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 Kohn-Sham nonlinear equations must be solved self-consistently, which often proves to be a frustrating task. Conventionally, the KS equations are solved iteratively using the self-consistent field (SCF) approach whose effectiveness is increased by introducing density mixing schemes. However, for some metallic and large systems, convergence is still difficult, and some guesswork is required to choose mixing parameters that coax the system towards the ground state. We found that propagating the electronic system in imaginary time using the time-dependent DFT framework can be useful in these problematic systems, owing to the continuous change in density which follows a trajectory with monotonically decreasing energy. Furthermore, the “charge sloshing” phenomenon often responsible for SCF non-convergence does not occur in imaginary time propagation. We present a few sample systems where this method outperforms standard SCF procedures. |
Wednesday, March 7, 2018 8:12AM - 8:24AM |
K02.00002: Construction of Interatomic Potentials and Mechanical Loss in Nb_{2}O_{5} Maher Yazback , Chris Billman , Alec Mishkin , Hai-Ping Cheng A BKS-Morse empirical potential is constructed for the B and H phases of Niobium Pentoxide (Nb_{2}O_{5}) for molecular dynamics simulation and energy landscape characterizations. Parameters of the interatomic potentials were adjusted to reproduce elastic properties of the two phases of Nb_{2}O_{5} simultaneously. Given the absence of experimental data, we compared these elastic properties to those obtained using density function theory (DFT) calculations. We applied the potential to study amorphous Nb_{2}O_{5} with structures generated by melting-quench classical molecular dynamics (MD) simulations. We then systematically located energy barriers for two-level systems. The energy barrier distribution collected from ~10^{4} barriers can be compared to experiments. We use the distribution function and calculated relaxation time, mechanical coupling constants, etc. to obtain the mechanical loss (Q^{-1}), which is of importance to optical coating materials for the future generation of gravitational wave detection. |
Wednesday, March 7, 2018 8:24AM - 8:36AM |
K02.00003: Spin–lattice simulations of cobalt and iron–cobalt alloys with LAMMPS Julien Tranchida , Aidan Thompson , Pascal Thibaudeau , Attila Cangi , Todd Monson , Eric Langlois , Jamin Pillars We performed a parallel implementation of spin–lattice dynamics into Sandia’s LAMMPS molecular dynamics code. The approach augments the phase space with a classical spin vector to each magnetic atom, in addition to its position and momentum. |
Wednesday, March 7, 2018 8:36AM - 8:48AM |
K02.00004: The MULTIBINIT software project Alexandre Martin , Jordan Bieder , Serguei Prokhorenko , Philippe Ghosez Although extremely powerful, DFT calculations are still limited to relatively |
Wednesday, March 7, 2018 8:48AM - 9:00AM |
K02.00005: Electrical Percolation in Multicomponent Composites: Monte Carlo Simulation and Experiment Xiaojuan Ni , Chao Hui , Ninghai Su , Raymond Cutler , Feng Liu A three-dimensional continuum percolation model has been developed based on Monte Carlo simulation to investigate the percolation behavior of an electrically insulating matrix reinforced with multiple conductive fillers of different dimensionalities. The model is able to predict the electrical percolation threshold in such multicomponent composites. The main finding is that a combination of multiple fillers with different dimensionalities can achieve a reduced working concentration below the percolation threshold of each single component, which has been confirmed by experimental results. Furthermore, impenetrable fillers of large aspect ratio preferentially align with each other to maximize the packing entropy rather than forming percolating cluster of randomly oriented fillers, and this entropy-driven transition from isotropic to nematic phase is found to affect the percolation threshold besides the concentrations of fillers. It suggests that the overall alignment of fillers characterized by a smaller nematic order parameter leads to a lower percolation threshold. |
Wednesday, March 7, 2018 9:00AM - 9:12AM |
K02.00006: Correct initialization and update method of the local simulation for electrostatic interactions Xiaozheng Duan , Issei Nakamura The local simulation algorithm for the electrostatic interactions, proposed by Maggs and Rossetto, serves as a fast simulation technique to consider electrolytes. The algorithm draws upon the auxiliary field variables on the lattice and may involve two topologically disconnected update schemes: the closed-loop and straight-line plaquette updates. We show that the straight-line update leads to unphysical configurations because the electrostatic field and system energy tend to diverge as the system size approaches zero. Such an unphysical state can be quickly decoupled from the observables by combining the closed-loop and straight-line updates. Alternatively, the simulation without the straight-line update becomes accurate when the initial configuration of the auxiliary field variables does not form the string-like structure on the lattice. We thus discuss an appropriate initial condition and an update method. |
Wednesday, March 7, 2018 9:12AM - 9:24AM |
K02.00007: Information based structure determination of amorphous carbon Bishal Bhattarai , Anup Pandey , Parthapratim Biswas , David Drabold In this talk, we offer large and realistic models of amorphous carbon spanning densities from 0.95 g/cc to 3.5 g/cc . The models are designed to agree as closely as possible with experimental diffraction data while simultaneously attaining a local minimum of a density functional Hamiltonian. The structure varies dramatically from interconnected wrapped and defective sp2 sheets at 0.95 g/cc to a nearly perfect tetrahedral topology at 3.5 g/cc . Force Enhanced Atomic Refinement (FEAR) was used and is shown here to be computationally superior and more experimentally realistic than conventional ab initio melt quench methods. We thoroughly characterize our models by computing structural, electronic and vibrational spectra. The vibrational density of states of the 0.95 g/cc model is strikingly similar to monolayer amorphous graphene. Our sp2 /sp3 ratios are close to experimental predictions where available, a consequence of compelling a satisfactory fit for pair correlation function. |
Wednesday, March 7, 2018 9:24AM - 9:36AM |
K02.00008: Design of Metamaterials for Mass Diffusion Juan Restrepo-Flórez , Martin Maldovan In recent years it has been proved that Fick’s law of diffusion is invariant under coordinate transformations therefore this theory can be used to design metamaterials to control mass diffusion. The main limitation of the Fick’s law approach is that the concentration gradient is considered as the driving force rather than the chemical potential, which governs mass transport. As a result, an implicit assumption is that the solubility of the diffusing species is constant throughout the system. This assumption is not applicable to heterogeneous systems, thus imposing additional constraints in the design of mass diffusion metamaterials. In this work we discuss how general metamaterial theory and effective medium formulations based on the chemical potential allows for the relaxation of constant solubility assumptions enabling the use of materials with different solubilities in mass diffusion metamaterials. By using this novel formulation it is possible to design metamaterials that simultaneously control the diffusion of several chemical species independently. Our results show that the use of a general metamaterials theory can enhance membrane-based separations in chemical and biochemical processes with improvements in performance orders of magnitude larger than standard membranes. |
Wednesday, March 7, 2018 9:36AM - 9:48AM |
K02.00009: On design of molecular photo-triggers for photochemical decomposition in high explosives Maxim Makeev Manipulation of chemical composition is a potentially efficient pathway for controlled ignition via the mechanism of photolysis in excited electronic states. Computational design methods can be used to find an optimal choice of trigger moiety – a chemical group attached to a high explosive to initiate decomposition reactions. We perform a quantum chemistry study of isolated RDX molecules chemically substituted by a series of aromatic moieties, with the aim of finding general recipes for molecular design of decomposition reaction triggers. Using the Wiberg bond indices as a comparative measure of bond strength, we study the effects of chemical composition and geometric particulars of the dyadic systems on the optical response properties and bond energies. The relation of these properties to the problem of ignition is discussed. |
Wednesday, March 7, 2018 9:48AM - 10:00AM |
K02.00010: Improvement of accuracy of wave-function-matching technique for first-principles electron-transport calculation Tomoya Ono , Shigeru Tsukamoto The wave-function-matching (WFM) technique for first-principles transport-property calculations was modified by Sørensen et al. so as to exclude rapidly decreasing evanescent waves [Sørensen et al., Phys. Rev. B 77, 155301 (2008)]. However, this method lacks translational invariance of the transmission probability with respect to insertion of matching planes and consistency between the sum of the transmission and reflection probabilities and the number of channels in the transition region. We reformulate the WFM method since the original methods are formulated to include all the generalized Bloch waves and find that the translational invariance is destroyed by the pseudoinverses used to exclude the rapidly decreasing evanescent waves. We then devise a method that calculates the transmission probability without the pseudoinverses. As a result, we find that the translational invariance is properly retained. In addition, we prove that the accuracy in the transmission probability of this WFM technique is comparable with that obtained by the nonequilibrium Green's function method. As an example of the proposed method, we introduce an application for the electron transport property of nanostructures. |
Wednesday, March 7, 2018 10:00AM - 10:12AM |
K02.00011: Elucidating the cluster versus crystal growth of the Solid Electrolyte Interphase components Asma Marzouk , Fernando Soto , Kie Hankins , Perla Balbuena , Fadwa El-Mellouhi The complexity of the Solid Electrolyte Interphase (SEI) at Li-ion batteries on graphitic electrodes, has triggered extensive deal of research due to its crucial properties for the long life of the battery. The SEI layer is composed by organic and inorganic species and results from the electrolyte decomposition on the electrode upon the first cycling of the battery. A stable SEI layer is crucial to maintain the chemical and mechanical stability of the electrode and the electrochemical stability of the electrolyte in order to prevent the irreversible capacity loss. |
Wednesday, March 7, 2018 10:12AM - 10:24AM |
K02.00012: Enhancing the predictive capability of modified Paschen curve models for direct current field emission assisted (FEA) microdischarges Jiba Dahal , Venkattraman Ayyaswamy Traditional plasma systems typically operate at low pressures and centimeter scale reactors. However, during the last decade or so, there is an active interest in the downscaling of devices and plasmas are no exception. Because of the popular pd (pressure times gap size) scaling, such plasmas have to operate at or near atmospheric pressure and are referred to as microplasmas or microdischarges. While downscaling the plasma device, field emission electrons from the field emission and their interaction with micro discharge due to high electric fields has shown to affect for both pre-and post breakdown. In this context, we are presenting the better understanding of field enhancement factor (β_{eff}) with electric field that able to predict the breakdown voltage in the microscale gap sizes. It is found that the β_{eff }is decreasing with increase in the electric field. The interpretation of experimental data using one-dimensional modified Paschen law indicates the similar electric field dependence. We have extracted the values of β_{eff} from different experimental datasets for microscale breakdown of different gases argon, hydrogen, Carbon-Dioxide and dry air analyze them and are shown to be consistent and an empirical dependence on electric field. |
Wednesday, March 7, 2018 10:24AM - 10:36AM |
K02.00013: Enhancing the electrode potential in Li-ion battery through atomic cluster approach Anoop Kushwaha Li-ion battery has wide area of application starting from low power consumer electronics to high power electric vehicles. However, their large scale application in electric vehicles are required further improvement due to their low specific power density. Several studies have found that related parameters can be taken care of by considering different cathode/anode materials and electrolytes. Recently, a noble approach has been reported on the basis of cluster size in which the use of Li_{3} cluster has been suggested as a potential component of the battery electrode material. In the present work, we have worked towards the improvement in the electrode potential of Li_{3} cluster with various electrolytes. Using the Ab initio quantum chemical calculation and the dielectric continuum model, the ionization potential and free energy of Li_{n }(n ≤ 8) cluster have been studied in both gas phase as well as in the dielectric solvent medium. We found that Li_{3 }exhibits the highest electrode potential among clusters in all the used solvents and the ethylene and propylene carbonate are found to show further improvement in the electrode potential. The enhancement in the potential is related to the ionization energy and difference of the solvation energy of the lithium clusters. |
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