Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J1: Recent Advances in Density Functional Theory IV |
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Sponsoring Units: DCP DCOMP Chair: Neepa Maitra, Hunter College Room: 103/105 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J1.00001: Towards accurate density-functional treatment of non-covalent interactions in complex systems Invited Speaker: Erin Johnson Accurate treatment of intermolecular interactions is an outstanding challenge in density-functional theory. While numerous approaches are now capable of modeling small molecular dimers, it is unclear how applicable these methods are to larger systems. This talk systematically investigates errors in density-functional approximations for non-covalent interactions, applied to both dispersion-bound and hydrogen-bonded clusters. In particular, a combination of the large-gradient behaviour of the exchange functional and delocalization error, rather than three-body dispersion terms, are shown to determine the performance of a given method. Additionally, we attempt to develop an optimal range-separated hybrid functional to pair with the exchange-hole dipole (XDM) dispersion model for molecular clusters. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J1.00002: Optimization of van der Waals Density Functionals using Data Projection onto Parameter Space (DPPS) Michelle Fritz, Marivi Fernandez-Serra, Mike Gillan, Jose M. Soler The parameterization and optimization of complex models fitted to reproduce a reference data set is an important part of the development of interatomic potentials. It is an approach that can also be used to design exchange and correlation functionals in density functional theory. Generally, this is a problem that requires choosing functional forms that depend on many parameters. The balance between the number of parameters and the size of the fitted data sets involves difficult and subjective decisions that are nevertheless critical for obtaining good results. We present a general and powerful optimization scheme, data projection onto parameter space (DPPS). The DPPS method tries to find the optimal parameters for a complex model which depends on a scalar function F which is determined by a large number of variables and parameters. The procedure involves the projection a vector of unknown parameters onto the vectors of known data. As an example, we apply DPPS to the optimization of the local exchange in a vdW density functional (vdW-DF). Our goal is to obtain an improved vdW-DF for water. To do so, we use an accurate potential energy surface for the water dimer as our initial data set. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J1.00003: Electronic Properties of Surfaces and Interfaces with Self-Consistent van der Waals Density Functional Nicola Ferri, Robert A. DiStasio Jr., Roberto Car, Alexandre Tkatchenko, Matthias Scheffler The long-range van der Waals (vdW) energy is only a small part of the total energy, hence it is typically assumed to have a minor influence on the electronic properties. Here, we address this question through a fully self-consistent (SC) implementation of the Tkatchenko-Scheffler (TS) density functional [1]. The analysis of TS-vdW$^{\rm{SC}}$ effects on electron density \textit{differences} for atomic and molecular dimers reveals quantitative agreement with correlated densities obtained from ``gold standard'' coupled-cluster quantum-chemical calculations. In agreement with previous work [2], we find a very small overall contribution from self-consistency in the structure and stability of vdW-bound molecular complexes. However, TS-vdW$^{\rm{SC}}$ (coupled with PBE functional) significantly affects electronic properties of coinage metal (111) surfaces, leading to an increase of up to 0.3 eV in the workfunction in agreement with experiments. Furthermore, vdW interactions visibly influence workfunctions in hybrid organic/metal interfaces, changing Pauli push-back and charge transfer contributions. [1] A. Tkatchenko and M. Scheffler, PRL (2009); [2] T. Thonhauser \textit{et al.}, PRB (2007). [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J1.00004: Van der Waals Interactions: Beyond Energies Alexandre Tkatchenko The strong and ubiquitous influence of van der Waals (vdW) interactions on the structure and stability of molecules and materials is well established by now. However, much less is known about the role of vdW interactions in electronic and response properties of molecules, solids, and interfaces between them. We have recently developed and coded a fully self-consistent implementation of the Tkatchenko-Scheffler vdW density functional, enabling us to categorically assess the role of long-range vdW interactions beyond trivial energetic stabilization. We demonstrate that vdW interactions have a significant impact (and improve agreement with experiment) for HOMO-LUMO gaps, dipole moments, and polarizabilities of ``chemically bound'' alkali dimers. We rationalize this result based on Feynman's view on vdW interactions arising from electrostatic-like picture [1] rather than from the more conventional electrodynamic model. Finally, the role of vdW interactions on workfunctions and charge transfer in hybrid organic/metal interfaces, as well as elastic properties of molecular materials will be shortly discussed. [1] R. P. Feynman, Phys. Rev. 56, 340 (1939). [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 4:18PM |
J1.00005: Recent Developments in Fragment-based Density Functional Theory Invited Speaker: Adam Wasserman I describe our progress on the development, implementation, and application of partition density functional theory (PDFT), a formally exact method for obtaining molecular properties from self-consistent calculations on isolated fragments [1]. For a given choice of fragmentation, PDFT outputs the (in principle exact) molecular energy and density, as well as fragment densities that sum to the correct molecular density. I discuss the behavior of the fragment energies as a function of fragment occupations [2], the different ways in which PDFT can be used to avoid the delocalization and static-correlation errors of approximate density functionals [3], our recent extension to the time-dependent case [4], and future directions. \\[4pt] [1] P. Elliott, K. Burke, M.H. Cohen, and A. Wasserman, Phys. Rev. A 82, 024501 (2010).\\[0pt] [2] R. Tang, J. Nafziger, and A. Wasserman, Phys. Chem. Chem. Phys. 14, 7780 (2012).\\[0pt] [3] J. Nafziger and A. Wasserman, arXiv:1305.4966 \\[0pt] [4] M. Mosquera, D. Jensen, and A. Wasserman, Phys. Rev. Lett. 111, 023001 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J1.00006: Ab-initio Charge and Spin Dynamics in Solids using TDDFT Peter Elliott, K Krieger, S Sharma, J.K Dewhurst, E.K.U Gross With the advent of ultrafast and high intensity laser pulses, we can probe many new and interesting phenomena. Due to the time-scale of such situations, fully quantum mechanical approaches for the electron dynamics are required. Time-dependent density functional theory (TDDFT) is the natural choice for this problem, as it balances accuracy and efficiency. Here we report on the implementation of real time TDDFT for periodic systems including non-collinear magnetization, in the ELK electronic structure code[1]. This allows us to study situations beyond the usual linear-response, for example ultrafast demagnetization[2] or laser-induced dielectric breakdown. Additionally, we are developing and testing new methods related to time-dependent problems, such as an exchange-correlation magnetic field which is locally non-collinear[3], a time-dependent polarization field, and coupling to Maxwell's equations. [1] elk.sourceforge.org [2] Ab-initio Ultrafast Demagnetization in Solids, K. Krieger, P. Elliott, S. Sharma, J.K. Dewhurst, E.K.U. Gross, in prep (2013). [3] Transverse spin-gradient functional for noncollinear spin-density-functional theory, F.G. Eich and E.K.U. Gross, Phys. Rev. Lett. 111, 156401 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J1.00007: Ensemble treatment of fragments within a molecule leads to improved description of dissociation Jonathan Nafziger, Adam Wasserman Approximate XC-functionals in Kohn-Sham (KS) density-functional theory (DFT) often fail when describing dissociation processes. This is due to improper treatment of fractional charges and spins within the dissociating systems. We demonstrate how the alternative framework of partition density-functional theory (PDFT) can correctly describe bond dissociation through its ensemble treatment of fragments within molecules. This method is illustrated through calculations on the dissociation of diatomic molecules. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J1.00008: Practical methods in time-dependent density functional theory (TDDFT) at elevated temperatures Rudolph Magyar, Luke Shulenburger, Andrew Baczewski There is a great need to simulate dynamic material response properties under shock conditions where experimental data is often limited due to the extreme scales involved (MBars, 1000s of K, and manifold compressed solid densities). Knowing materials properties at this scale is vital element of simulations of planetary collisions, inertial confinement fusion experiments, and the surfaces of some stars. Considerable progress has been made using density functional molecular dynamics (DFT-MD) to model thermodynamic properties of material under these conditions; however, the approach is limited to cases in which the electrons are constrained to a thermodynamic distribution within the Mermin formulation. We will explore practical schemes to generalize this method to the time-dependent case. Several challenges come up such as the role of non-adiabatic electron-electron and electron-nuclear physics and the correct choice of initial state. One of the most straightforward choices of initial state is to project the Mermin state since the original Runge-Gross proof does not make explicit choice of occupations. We will present some numerical tests of finite systems to examine this formulation. We will also explore how simple models of non-adiabatic effects might be sufficiently accurate under extreme conditions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J1.00009: Recovering the Integer Discontinuity of Density Functional Approximations Martin Mosquera, Adam Wasserman The derivative discontinuity (DD) of the exchange-correlation (XC) energy of density functional theory (DFT) is a consequence of the piece-wise linear dependency of the energy functional on the number of electrons of atoms or fragments that have been separated adiabatically from a molecule. Most approximations to the XC energy functional as the local-density approximation, the generalized-gradient approximation, exact exchange, among others, miss the DD or the piece-wise linear behavior, leading to inconsistencies in the analysis of molecular dissociation. We derive formal properties of the {\slshape exact} XC energy functional that lead to a framework to correct {\slshape any} density-functional approximation to display the required piece-wise linear dependency on the number of electrons and the DD. We will also illustrate how new approximation to the XC energy functionals can be developed for applications in DFT and fragment-based extensions. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J1.00010: Modeling long-range time-resolved charge-transfer within TDDFT: Insights from a 2-site lattice model Johanna Fuks, Neepa Maitra It has been shown that approximate adiabatic TDDFT functionals dramatically fail to reproduce time-resolved long-range charge-transfer dynamics (LR-CTD) [1]. In order to decouple the impact of the adiabatic approximation and the choice of ground state (gs) functional it would be instructive to propagate using the adiabatically-exact (adia-ex) functional. Numerically this involves an iterative process at each time-step to find the gs potential for a given density, which converges badly for CTD due to regions of low density. To circumvent this, we use as model system an asymmetric 2-site Hubbard model with small hopping parameter, its small Hilbert space allows to perform a Levy-Lieb constrained search and find the exact gs Hartree-exchange-correlation (Hxc) functional [2]. The later develops a sharp step feature in the long-range limit (limit of small hopping parameter). Both closed-shell to closed-shell and open-shell to open-shell LR-CT are investigated. By propagating the Kohn-Sham system in the presence of the exact gs Hxc functional under a resonant laser we are able to perform, for the first time, a fully self-consistent adia-ex propagation for CTD. [1] J. I. Fuks, P Elliott, A. Rubio, N. T. Maitra, JPCLett 4, 735 - 739 (2013) [2] J. I. Fuks et al, in preparation [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J1.00011: Time-Resolved Dynamics in Exact TDDFT: Studies of Two-Electron Systems Ernesto Sandoval, Johanna Fuks, Kai Luo, Neepa Maitra, Peter Elliott An exact decomposition of the exchange-correlation potential in time-dependent density functional theory (TDDFT) into kinetic and hole contributions is derived, with the goal of a better understanding of features of the TDDFT functionals, leading eventually to improved approximations. We study the kinetic and hole contributions for a range of dynamical situations in two-electron systems in one-dimension, from models of Rabi oscillations to local excitations, charge-transfer excitations, and resonance energy transfer, and compare them to their adiabatically-exact approximation. We find that dynamical step structures are present in both terms, that require a non-adiabatic functional approximation. In many cases, the kinetic contribution dominates the step structure, but not in all. The adiabatically-exact approximation is generally worse for the kinetic contribution than for the hole contribution. [Preview Abstract] |
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