Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K31: Advances in Density Functional Theory VFocus

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Sponsoring Units: DCP Chair: Michele Pavanello, Rutgers University Room: 331 
Wednesday, March 16, 2016 8:00AM  8:36AM 
K31.00001: \textbf{Ultrafast LaserInduced Demagnetization: Identifying the Mechanism with RealTime TDDFT} Invited Speaker: E.K.U. Gross In the past 2 decades several experiments have demonstrated the laserinduced demagnetization of ferromagnetic solids in less than 100 femtoseconds. This is orders of magnitude faster than the presentday magneticfieldbased technology. To shed light on the underlying microscopic mechanism, we have performed an abinitio study of Fe, Co, Ni and Cr in short laser pulses, using realtime noncollinear timedependent spin density functional theory (TDDFT). We show [1] that the demagnetization proceeds in two distinct steps: First, a fraction of the electrons is excited without much change in the total spin polarization. In a second step, the spin magnetic moment of the remaining localized delectrons decreases through spinflip transitions induced by spinorbit coupling. For pulse lengths of a few femtoseconds, the whole process of demagnetization happens in less than 50 femtoseconds. For antiferromagnetic Heusler compounds, such as Mn$_{\mathrm{3}}$Ga and Ni$_{\mathrm{2}}$MnGa, an even faster process is found where magnetic moment is transferred from one sublattice to the other. Employing a combination [2] of TDDFT with Optimal Control Theory, we furthermore demonstrate how the demagnetization process can be controlled with suitably shaped laser pulses. Finally, we assess the influence of the approximation used for the exchangecorrelation (xc) functional by comparing noncollinear LSDA results with a novel xc functional [3] that exerts a local exchangecorrelation torque. \begin{enumerate} \item K. Krieger, J.K. Dewhurst, P. Elliott, S. Sharma, E.K.U. Gross, JCTC\textbf{ 11, }4870 (2015). \item A. Castro, J. Werschnik, E.K.U. Gross, Phys. Rev. Lett. \textbf{109}, 153603 (2012). \item F.G. Eich, E.K.U. Gross, Phys. Rev. Lett. \textbf{111}, 156401 (2013). \end{enumerate} [Preview Abstract] 
Wednesday, March 16, 2016 8:36AM  8:48AM 
K31.00002: Realtime dynamics of Hubbardtype model systems via a combination of the KadanoffBaym formalism with adiabatic TDDFT Hopjan Miroslav, Daniel Karlsson, SImon Ydman, Claudio Verdozzi, CarlOlof Almbladh We propose a description of nonequilibrium systems via a simple protocol that combines DFTexchangecorrelation potentials with selfenergies of manybody perturbation theory. The approach, aimed to avoid double counting of interactions, is tested against exact results in Hubbardtype systems, with respect to interaction strength, perturbation speed/inhomogeneity, and system dimensionality/size. In many regimes, we observe good agreement with the exact results, and an improvement over the pure adiabatic local TDDFT or the pure SecondBorn NEGF approximations. We also address the reasons behind the residual discrepancies, and briefly discuss possible directions for future work. [Preview Abstract] 
Wednesday, March 16, 2016 8:48AM  9:00AM 
K31.00003: Electronic stopping in liquid water from first principles: An application of largescale realtime TDDFT simulations Kyle Reeves, Yi Yao, Yosuke Kanai Electronic stopping describes the transfer of energy from a highlyenergetic charged particle to electrons in a material. This process induces massive electronic excitations via interaction between the material and the highly localized electric field from the charged particle. Understanding this phenomenon in condensed matter systems under proton irradiation has implications in various modern technologies. Firstprinciples simulations, based on our recentlydeveloped largescale realtime timedependent density functional theory approach, provide a detailed description of how electrons are excited via a nonequilibrium energy transfer from protons on the attosecond time scale. We apply this computational approach to the important case of liquid water under proton irradiation. Our work reveals several key features of the excitation dynamics at the mesoscopic and molecular levels which support a clearer understanding of the water radiolysis mechanism under proton irradiation. Importantly, we will demonstrate a firstprinciples determination of the energy transfer rate, (i.e. electronic stopping power) in liquid water, and a comparison to existing empirical models will be presented. We will conclude by discussing how the exchangecorrelation approximation influences the calculation of the electronic stopping power. [Preview Abstract] 
Wednesday, March 16, 2016 9:00AM  9:12AM 
K31.00004: Electrical conductivity of metals from realtime timedependent density functional theory Xavier Andrade, Alfredo Correa In this presentation, I will discuss how to apply realtime electron dynamics to study electronic currents in crystalline systems and, in particular, how to use this method to predict electrical conductivities in different regimes. This approach presents many interesting theoretical challenges associated to the representation of bulk systems as infinitely periodic. For example, in order to induce electronic currents in the system, we use a gauge transformation that allows us to include finite electric fields in the simulation. We have implemented this approach using timedependent density functional theory (TDDFT). This implementation allows us to induce, measure and visualize the current density as a function of time, in simulations with thousands of electrons (hundreds and even thousands of atoms). We have found that realtime TDDFT can describe how currents naturally decay in metals. From this dissipation process we can directly calculate the frequencydependent conductivity, including the direct current (DC) conductivity that is not accessible from linearresponse approaches like KuboGreenwood. [Preview Abstract] 
Wednesday, March 16, 2016 9:12AM  9:24AM 
K31.00005: Development of an abinitio calculation method for 2D layered materialsbased optoelectronic devices Han Seul Kim, YongHoon Kim We report on the development of a novel firstprinciples method for the calculation of nonequilibrium nanoscale device operation process. Based on regiondependent $\Delta $ selfconsistent field method beyond the standard density functional theory (DFT), we will introduce a novel method to describe nonequilibrium situations such as external bias and simultaneous optical excitations. In particular, we will discuss the limitation of conventional method and advantage of our scheme in describing 2D layered materialsbased devices operations. Then, we investigate atomistic mechanism of optoelectronic effects from 2D layered materialsbased devices and suggest the optimal material and architecture for such devices. [Preview Abstract] 
Wednesday, March 16, 2016 9:24AM  9:36AM 
K31.00006: The TimeDependent ParticleHole Map Carsten Ullrich, Yonghui Li The particlehole map (PHM) is proposed as a new visualization tool to analyze electronic excitations in molecules in the time or frequency domain, to be used in conjunction with TDDFT or other ab initio methods. The purpose of the PHM is to give detailed insight into electronic excitation processes which is not obtainable from local visualization methods such as transition densities, density differences, or natural transition orbitals. The PHM provides information on the origins, destinations, and coherences of charge fluctuations during an excitation process. In contrast with the transition density matrix, the PHM has a statistical interpretation involving joint probabilities of individual states and their transitions, and it is easier to read and interpret. We discuss and illustrate the properties of the PHM and give several examples and applications to excitations in onedimensional model systems and in organic donoracceptor systems. [Preview Abstract] 
Wednesday, March 16, 2016 9:36AM  9:48AM 
K31.00007: Exploiting initialstate dependence to improve the performance of adiabatic TDDFT Johanna I. Fuks, Soeren E.B. Nielsen, Michael Ruggenthaler, Neepa T. Maitra Although timedependent density functional theory (TDDFT) descriptions of dynamics in nonequilibrium situations have seen exciting successes recently, there have also been studies that throw into doubt the reliability of the approximate exchangecorrelation functionals to accurately describe the dynamics.~ Here we study exact exchangecorrelation potentials for few electron systems, found using the global fixedpoint iteration method [NRL]. We find that the size of dynamical correlation features that are missing in the currentlyused adiabatic approximations depend strongly on the choice of the initial KohnSham wavefunction. With a judicious choice, the dynamical effects can be small over a finite time duration, but sometimes they can get large at longer times.~ We also examine different starting points, in particular an orbitaldependent potential directly obtained from the KohnSham hole [LFSEM14], ~for approximate xc functionals: instead of building on an adiabatic approximation. [Preview Abstract] 
Wednesday, March 16, 2016 9:48AM  10:00AM 
K31.00008: Studies of spuriously timedependent resonances in TDDFT Neepa Maitra, Johanna Fuks Recently the failure of some exchangecorrelation functionals to accurately ~capture nonperturbative dynamics in timedependent density functional theory (TDDFT) was shown to be correlated with their violation of an exact condition [1]: that the resonance positions of the system remain fixed during the evolution.~ Closely related is the inconsistent prediction of excitation frequencies in linear response when the reference state is an excited state of the system. We discuss this and the effect on dynamics in a range of molecular systems, exploring systemsize dependence of the violation. [1] ~Timeresolved spectroscopy in timedependent density functional theory: An exact condition, J. I. Fuks, K. Luo, E. D. Sandoval, N. T. Maitra, Phys. Rev. Lett.~\textbf{114}, 183002 (2015).~ [Preview Abstract] 
(Author Not Attending)

K31.00009: Open Quantum Transport and NonHermitian RealTime TimeDependent Density Functional Theory Justin Elenewski, Yanxiang Zhao, Hanning Chen Subnanometer electronic devices are notoriously difficult to simulate, with the most widely adopted transport schemes predicting currents that diverge from experiment by several orders of magnitude. This deviation arises from numerous factors, including the inability of these methods to accommodate dynamic processes such as charge reorganization. A promising alternative entails the direct propagation of an electronic structure calculation, as exemplified by realtime timedependent density functional theory (RTTDDFT). Unfortunately this framework is inherently that of a closed system, and modifications must be made to handle incoming and outgoing particle fluxes. To this end, we establish a formal correspondence between the quantum master equation for an open, manyparticle system and its description in terms of RTTDDFT and nonHermitian boundary potentials. By dynamically constraining the particle density within the boundary regions corresponding to the device leads, a simulation may be selectively converged to the nonequilibrium steady state associated with a given electrostatic bias. Our numerical tests demonstrate that this algorithm is both highly stable and readily integrated into existing electronic structure frameworks [Preview Abstract] 
Wednesday, March 16, 2016 10:12AM  10:24AM 
K31.00010: TDDFTbased local control theory for chemical reactions Ivano Tavernelli, Basile F. E. Curchod, Thomas J. Penfold In this talk I will describe the implementation of local control theory for laser pulse shaping within the framework of TDDFTbased nonadiabatic dynamics~\footnote{B. Curchod, T. Penfold, U. Rothlisberger, I. Tavernelli, Phys. Rev. A, 84, 042507, 2011}. The method is based on a set of modified Tully’s surface hopping equations and provides an efficient way to control the population of a selected reactive state of interest through the coupling with an external timedependent electric field generated onthefly during the dynamics. This approach is applied to the investigation of the photoinduced intramolecular proton transfer reaction in 4hydroxyacridine in gas phase and in solution~\footnote{B.F.E. Curchod, T. J Penfold, U. Rothlisberger, I.Tavernelli, Chem. Phys. Chem., 16, 2127, 2015}. The generated pulses reveal important information about the underlying excitedstate nuclear dynamics highlighting the involvement of collective vibrational modes that would be neglected in studies performed on model systems. Finally, this approach can help to shed new light on the photophysics and photochemistry of complex molecular systems and guide the design of novel reaction paths. [Preview Abstract] 
Wednesday, March 16, 2016 10:24AM  10:36AM 
K31.00011: Mixed QuantumClassical Dynamics Methods for StrongField Processes: Multipletrajectory Ehrenfest dynamics $+$ decoherence terms Yasumitsu Suzuki, Kazuyuki Watanabe, Ali Abedi, Federica Agostini, Seung Kyu Min, Neepa Maitra, E. K. U. Gross The exact factorization of the electronnuclear wave function [1, 2, 3] allows to define the timedependent potential energy surfaces (TDPESs) responsible for the nuclear dynamics and electron dynamics. Recently a novel coupledtrajectory mixed quantumclassical (CTMQC) approach based on this TDPES has been developed [4], which accurately reproduces both nuclear and electron dynamics. Here we study the TDPES for laserinduced electron localization with a view to developing a MQC method for strongfield processes [5]. We show our recent progress in applying the CTMQC approach to the systems with many degrees of freedom. [1] A. Abedi, N. T. Maitra, E. K. U. Gross, Phys. Rev. Lett. 105, 123002 (2010). [2] Y. Suzuki, A. Abedi, N. T. Maitra, K. Yamashita, E. K. U. Gross, Phys. Rev. A, 89, 040501(R) (2014). [3] A. Abedi, F. Agostini, Y. Suzuki, E. K. U. Gross, Phys. Rev. Lett. 110, 263001 (2013); F. Agostini, A. Abedi, Y. Suzuki, S. K. Min, N. T. Maitra, E. K. U. Gross, J. Chem. Phys., 142, 084303 (2015). [4] S. K. Min, F. Agostini, E. K. U. Gross, Phys. Rev. Lett.,115, 073001, (2015). [5] Y. Suzuki, A. Abedi, N. T. Maitra, E. K. U. Gross, Phys. Chem. Chem. Phys., 17, 29271 (2015). [Preview Abstract] 
(Author Not Attending)

K31.00012: Nuclear Quantum Effects in the Dynamics of Biologically Relevant Systems from First Principles Mariana Rossi, Wei Fang, Angelos Michaelides Understanding the structure and dynamics of biomolecules is crucial for unveiling the physics behind biologyrelated processes. These molecules are very flexible and stabilized by a delicate balance of weak (quantum) interactions, thus requiring the inclusion of anharmonic entropic contributions and an accurate description of the electronic and nuclear structure from quantum mechanics. We here join state of the art densityfunctional theory (DFT) and path integral molecular dynamics (PIMD) to gain quantitative insight into biologically relevant systems. Our design of a better and more efficient approximation to quantum time correlation functions based on PIMD (TRPMD [1,2]) enables the calculation of ab initio TCFs with which we calculate IR/vibrational spectra and diffusion coefficients. In stacked polyglutamine strands (structures often related to amyloid diseases) a combination of NQE and Hbond cooperativity provides a small free energy stabilization that we connect to a softening of high frequency modes, enhanced by nuclear quantum anharmonicity [3]. [1] Rossi, Ceriotti, Manolopoulos, JCP {\bf 140}, 234116 (2014); [2] Rossi et al., JCP {\bf 141}, 181101 (2014); Rossi, Fang, Michaelides, JPCL {\bf 6}, 4233 (2015) [Preview Abstract] 
Wednesday, March 16, 2016 10:48AM  11:00AM 
K31.00013: Properties of the Schr\"{o}dinger Theory for Electrons in External Fields Viraht Sahni, XiaoYin Pan We consider electrons in external electrostatic ${\boldsymbol{\cal{E}}} ({\bf{r}}) =  {\boldsymbol{\nabla}} v ({\bf{r}})$ and magnetostatic ${\bf{B}} ({\bf{r}}) = {\boldsymbol{\nabla}} \times {\bf{A}} ({\bf{r}})$ fields. (The case of solely an electrostatic field constitutes a special case.) Via the `Quantal Newtonian' first law for the individual electron we prove the following: (i) In addition to the external electric and Lorentz fields, each electron experiences an internal field representative of electron correlations due to the Pauli exclusion principle and Coulomb repulsion, the kinetic energy, the density, and the magnetic field; (ii) the scalar potential $v ({\bf{r}})$ arises from a curlfree field and is thus pathindependent; (iii) the magnetic field ${\bf{B}} ({\bf{r}})$ appears explicitly in the Schr\"{o}dinger equation in addition to the vector potential ${\bf{A}} ({\bf{r}})$; (iv) The Schr\"{o}dinger equation can be written to exhibit its intrinsic selfconsistent form. (The generalization of the conclusions to timedependent external fields via the `Quantal Newtonian' second law follows.) [Preview Abstract] 
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