Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B29: Firstprinciples Modeling of ExcitedState Phenomena in Materials II: Realtime TDDFTFocus

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Sponsoring Units: DCOMP DMP DCP DCMP Chair: Andre Schleife, University of Illinois at Urbana–Champaign Room: LACC 406A 
Monday, March 5, 2018 11:15AM  11:51AM 
B29.00001: Recent advances in timedependent density functional theory for applications to electronic excitations and nonadiabatic dynamics Invited Speaker: Efthimios Kaxiras Expert in LowDimensional Systems and TDDFT, which is part of our Focus Topic. 
Monday, March 5, 2018 11:51AM  12:03PM 
B29.00002: Long Time Behavior of TimeDependent Density Functional Theory Normand Modine , ChengWei Lee , Andre Schleife We investigate whether realtime TDDFT calculations can capture internal equilibration of an electronic system by performing multipicosecond TDDFT simulations of aluminum. We monitor the system by calculating the projections of the TDDFT states into the KohnSham eigenvectors of the system. For a system of noninteracting Fermions at equilibrium, the ensemble average of these projections would follow a Fermi distribution, but for interacting systems (e.g., a Fermi liquid), the distribution can be modified. Using a recently published algorithm (Modine and Hatcher, JCP 142, 204111 (2014)), we construct initial TDDFT states whose average projections match a Fermi distribution. During TDDFT propagation of these states, the calculated projections are observed to evolve away from the Fermi distribution to a modified distribution with a sharper drop at the Fermi level and longer tails at high and low energies. 
Monday, March 5, 2018 12:03PM  12:15PM 
B29.00003: Reproducibility in LRTDDFT: The Importance of The Continuum of Unoccupied Orbitals Maxime Morinière , Luigi Genovese , Thierry Deutsch The reproducibility of DFT results was addressed recently [1], and other areas of computational physics and chemistry could benefit from the same type of study. As a method building on top of DFT, LinearResponse TimeDependent DFT (LRTDDFT) [2] is a potential candidate for such studies. In a typical LRTDDFT calculation, the inclusion of the continuum of unoccupied orbitals is required, and we will see that this is not without raising difficulties when comparing the results of different codes. We will eventually show that the convergence of LRTDDFT spectra is only possible over a finite range of energy that depends on the system under consideration. This conclusion allows for a meaningful study of the reproducibility of LRTDDFT calculations. 
Monday, March 5, 2018 12:15PM  12:27PM 
B29.00004: Exponential Integrator Methods in TimeDependent Density Functional Calculations Kalman Varga , Daniel Kidd , Cody Covington The integrating factor and exponential time differencing methods are implemented and tested within onedimensional timedependent density functional theory. Popular time propagation methods used in physics are also tested and compared to these exponential integrator methods. We determine an improvement in accuracy of multiple orders of magnitude when describing dynamics driven by nonlinear potentials using fourthorder Runge–Kuttatype exponential integrators. For cases of dynamics driven by a timedependent external potential, the accuracy of the exponential integrator methods are less enhanced but still match or outperform the best of the conventional methods tested. 
Monday, March 5, 2018 12:27PM  12:39PM 
B29.00005: Realtime timedependent density functional theory using higherorder finiteelement methods Bikash Kanungo , Vikram Gavini We present a computationally efficient approach to solve the timedependent KohnSham (TDKS) equations in realtime using higherorder finiteelement spatial discretization, applicable to both pseudopotential and allelectron calculations. To this end, we develop an a priori mesh adaption technique, based on semidiscrete error estimate on the timedependent density, to construct a close to optimal finiteelement discretization. We employ spectral finiteelements along with GaussLegendreLobatto quadrature to render the overlap matrix diagonal, thereby simplifying the inversion of the overlap matrix that features in the evaluation of the discrete propagator. We demonstrate a staggering reduction in the computational time afforded by higherorder finiteelements over linear finiteelements. We also perform a comparative study of the computational efficiency of the proposed method against finite difference (FD) based method and Gaussian basis for pseudopotential and allelectron calculations, respectively. Lastly, we demonstrate the capability and scalability of the proposed method on various largescale systems. 
Monday, March 5, 2018 12:39PM  12:51PM 
B29.00006: Negative differential conductivity in liquid aluminum from realtime density functional theory Xavier Andrade , Sebastien Hamel , Alfredo Correa I this talk we will present our approach for the calculation of nonlinear electronic conductivy based on timedependent density functional theory (TDDFT). Based on these simulations, we predict that liquid aluminum exhibits negativedifferential conductivity for current densities of the order of 10^{12}−10^{13 }A/cm^{2}. We find that the changes in the conductivity, as the current increases, emerge from a competition between the accumulation of charge around the nuclei, that increases the scattering of the conduction electrons, and a decreasing scattering crosssection at high currents. 
Monday, March 5, 2018 12:51PM  1:03PM 
B29.00007: Projectile's Core Electrons and Large Values of Electronic Stopping Rafi Ullah , Emilio Artacho , Alfredo Correa Light projectiles (H, He) shooting through solids lose energy at a rate of the order of a few eV/Ang, this value peaking at projectile velocities around 1% of the speed of light. These processes are well reproduced in simulations of electronic stopping using timedependent densityfunctional theory in real time. Good agreement with nontrivial experimental results have already been obtained for a variety of hosts materials or liquids. Heavier projectiles as 3d transition metals, however, give rise to electronic stopping powers in the keV/Ang range, and the maximum stopping occurs at velocities several times higher. Here we present results of Ni projectiles shooting through bulk Ni using a planewave implementation of TDDFT(t). The effect of core electrons is investigated, finding that the explicit and flexible consideration of deep core states of the projectile is crucial for an accurate description of the problem. Remarkably, instantaneous semilocal TDDFT seems to capture the relevant physics in the electron excitation process relevant to electronic stopping. 
Monday, March 5, 2018 1:03PM  1:15PM 
B29.00008: Charge equilibration and electronic stopping for silicon projectiles in silicon ChengWei Lee , Andre Schleife Understanding the effect of highly energetic particle radiation on semiconductors from first principles is important, e.g. for radiation hardness as well as ion implantation to create quantum bits. Previously we successfully used Ehrenfest molecular dynamics and realtime timedependent density functional theory to describe electronic stopping during the early stages of radiation damage for light projectiles. Our recent results for heavier (silicon) projectiles traversing silicon bulk crystals show a pronounced dependence of electronic stopping on the initial condition, which we relate to the charge of the projectile. Not only was this effect absent for light projectiles, but on a femtosecond time scale this also depends on whether the target material is a metal or semiconductor. We analyze these recent results in terms of charge equilibration and contributions of core and valence electrons to electronic stopping. Developing a consistent picture from first principles is necessary to describe electronic friction in classical molecular dynamics with predictive accuracy. 
Monday, March 5, 2018 1:15PM  1:27PM 
B29.00009: Electronic stopping power of metallic clusters in TDDFT Ivan Maliyov , JeanPaul Crocombette , Fabien Bruneval Irradiation damage in condensed matter is central to many technological fields: from nuclear energy production to medical physics. The interaction between an irradiating ion and a target material is characterized by the energy transfer between them, named the stopping power. Most of the energy transfer is mediated through the electron excitations of the target. This is the electronic stopping power that we aim at calculating from ab initio simulations here. 
Monday, March 5, 2018 1:27PM  1:39PM 
B29.00010: Hotelectron mediated diffusion in protonirradiated MgO ChengWei Lee , Andre Schleife Ionizing radiation is known to give rise to enhanced defect diffusion in materials. However, existing models including "thermal spike'' and "ionizationenhanced diffusion'' focus on ion dynamics and neglect the influence of electronic excitations due to electronic stopping. We use timedependent density functional theory (TDDFT) in Ehrenfest dynamics simulations to describe the first 30 fs after proton impact into MgO directly from first principles. Comparison to BornOppenheimer molecular dynamics clearly illustrates the importance of electronic excitations for the emerging ion dynamics. In order to quantify the effect of hot electrons and the resulting modified ion dynamics on migration barriers, we combine TDDFT with constrained DFT and the nudgedelastic band method. Our data shows that hot electrons need to be described explicitly and, during the early stages after particle impact, cannot be approximated by a Fermi temperature or an effective charge state of defects in the material. Due to the large computational cost, these simulations are restricted to shorttime dynamics, however, we extract parameters that enable improved multiscale simulations based on twotemperature molecular dynamics and migration barriers in kinetic MonteCarlo studies. 
Monday, March 5, 2018 1:39PM  1:51PM 
B29.00011: Electronic Response of Graphene to Ion Irradiation Alina Kononov , Andre Schleife Graphene and other twodimensional materials have recently emerged as promising candidates for novel electronic devices. However, these applications often require highresolution imaging and processing techniques, which typically employ focused ion beams. Thus, achieving finer control of the structure and properties of graphene necessitates a detailed understanding of the excited electron dynamics occurring in the material in response to ion irradiation. Using realtime timedependent density functional theory and Ehrenfest dynamics, we simulate 1080 keV protons traversing monolayer and trilayer graphene. We calculate the secondary electron yield, charge transfer, energy transfer, and equilibration timescales after impact, and we investigate the dependence of these quantities on graphene thickness, projectile velocity, and projectile trajectory. We find that energy transfer is maximized with a proton energy near 80 keV, while electron emission and charge transfer are maximized with a proton energy near 25 keV. 
Monday, March 5, 2018 1:51PM  2:03PM 
B29.00012: Quadratic response properties from TDDFT: trials and tribulations Shane Parker , Dmitrij Rappoport , Filipp Furche Nonlinear response theory is an increasingly important theoretical tool used to compute, for example, nonlinear optical properties needed to characterize complex materials or electronic couplings between excited states in timedependent density functional theory (TDDFT). I will present an efficient implementation of the TDDFT quadratic response function, including the static and dynamic hyperpolarizability, twophoton absorption amplitudes, and excitedstate absorption amplitudes. The successes and failures (with respect to the quadratic response function's recently detailed incorrect pole structure) of existing nonlinear response methods will be sketched out by computing the dynamic hyperpolarizability of several steroisomers of octupolar calix[4]arenes, the twophoton absorption spectra of a series of twisted conjugated porphyrins, and the excitedstate absorption spectra during excimer formation in perylene diimide dimers. I will close with guidelines for practitioners on avoiding spurious resonances in applications, and by discussing possible remedies. 
Monday, March 5, 2018 2:03PM  2:15PM 
B29.00013: Realtime TDDFT simulation of nonlinear effects in the absorption of intense soft Xrays in solidstate systems Sri Chaitanya Das Pemmaraju Synchrotron based Xray spectroscopies are well described within linear response theory. The interaction with matter, of intense Xray pulses from modern Xray free electron lasers, can extend into the nonliner and nonperturbative regimes and requires theoretical descriptions that go beyond linear response. In this study the velocity gauge realtime TDDFT approach is employed to investigate nonlinear effects in the interaction of intense few femtoscond Xray pulses with solidstate materials. In particular the role of lightinduced transparency and stimulated emission in modulating the absorption signatures of XFEL pulses at light element Kedges and transition metal Ledges is explored through prototypical simulations on Silicon Carbide and metallic Cobalt. Absorption changes driven by nonlinear interactions are contrasted against those induced by lattice and electronic heating effects. Additionally, TDDFT results are compared to predictions from phenomenological optical bloch equation simulations. 
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