Bulletin of the American Physical Society
APS April Meeting 2013
Volume 58, Number 4
Saturday–Tuesday, April 13–16, 2013; Denver, Colorado
Session J15: Computational Physics I 
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Sponsoring Units: DCOMP Chair: Tim Germann, Los Alamos National Laboratory Room: Plaza Court 4 
Sunday, April 14, 2013 1:30PM  1:42PM 
J15.00001: Thermodynamic of cellulose solvation in novel solvent mixtures Ritankar Das, JhihWei Chu Biomass contains abundant amounts of cellulose as crystalline microfibrils. A limiting step to using cellulose as an alternative energy source, however, is the hydrolysis of the biomass and subsequent transformation into fuels. Cellulose is insoluble in most solvents including organic solvents and water, but it is soluble in some ionic liquids like BMIMCl. This project aims to find alternative solvents that are less expensive and are more environmentally benign than the ionic liquids. Allatom molecular dynamics simulations were performed on dissociated glucan chains separated by multiple (45) solvation shells, in the presence of several novel solvents and solvent mixtures. The solubility of the chains in each solvent was indicated by contacts calculations after the equilibration of the molecular dynamics. It was discovered that pyridine and imidazole acted as the best solvents because their aromatic electronic structure was able to effectively disrupt the intersheet interactions among the glucan chains in the axial direction, and because perturbation of the solvent interactions in the presence of glucan chains was minimal. [Preview Abstract] 
Sunday, April 14, 2013 1:42PM  1:54PM 
J15.00002: Dynamics and Interactions of Adsorbates on Palladium and Nickel Clusters Ajit Hira, Jose Pacheco, Justin Salazar, Clifton Brownrigg We continue our interest on the interactions of different atomic and molecular species with small clusters of metallic elements, by examining the interactions of H, O and F atoms with Pd$_{\mathrm{n}}$ and Ni$_{\mathrm{n}}$ clusters (n $=$ 6 thru 12). The hybrid ab initio methods of quantum chemistry (particularly the DFTB3LYP model) are used to derive optimal geometries for the clusters of interest. We compare calculated binding energies, bondlengths, ionization potentials, electron affinities and HOMOLUMO gaps for the clusters of the two different metals. Of particular interest are the comparisons of binding strengths at the three important types of sites: edge (E) sites, hollow sites (H) site and ontop (T) sites. Effects of crystal symmetries corresponding to the bulk structures for the two metals will also be investigated. Our theoretical results will be compared with the experimental studies where they are available. We will also study the dynamics of the atomic species, and the dynamics and dissociation of the molecular species on the clusters. [Preview Abstract] 
Sunday, April 14, 2013 1:54PM  2:06PM 
J15.00003: Timedependent density functional theory of extreme environments Luke Shulenburger, Michael Desjarlais, Rudolph Magyar We describe the challenges involved when using timedependent density functional theory (TDDFT) to describe warm dense matter (WDM) within a planewave, realtime formulation. WDM occurs under conditions of temperature and pressure (over 1000 K and 1 Mbar) where plasma physics meets condensed matter physics. TDDFT is especially important in this regime as it can describe ions and electrons strongly out of equilibrium. Several theoretical challenges must be overcome including assignment of initial state orbitals, choice of timepropogation scheme, treatment of PAW potentials, and inclusion of nonadiabatic effects in the potential energy surfaces. The results of these simulations are critical in several applications. For example, we will explain how the TDDFT calculation can resolve modeling inconsistencies in Xray Thompson crosssections, thereby improving an important temperature diagnostic in experiments. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DEAC0494AL85000. [Preview Abstract] 
Sunday, April 14, 2013 2:06PM  2:18PM 
J15.00004: KadanoffBaymKeldyshEhrenfest dynamics of correlated materials responding to ultrafast laser pulses Roland Allen In our many earlier simulations of the response of materials and molecules to laser pulses, oneelectron states were determined by the timedependent Schr\"{o}dinger equation with an instantaneous oneelectron Hamiltonian. These states were then used with Ehrenfest's theorem in a semiclassical treatment of the coupled dynamics of electrons and nuclear coordinates. For stronglycorrelated materials, however, true nonequilibrium selfenergies are required. Here we describe a practical numerical procedure for employing the KadanoffBaym/Keldysh equations\footnote{See e.g. A. Stan, N. E. Dahlen, and R. van Leeuwen, J. Chem. Phys. 130, 224101 (2009) and T. Kita, Prog. Theor. Phys. 123, 581 (2010).} together with Ehrenfest's theorem, and with conserving selfenergies appropriate to correlated materials. [Preview Abstract] 
Sunday, April 14, 2013 2:18PM  2:30PM 
J15.00005: Timedependence of electromagnetic selfinteractions of fermions in one dimension Athanasios Petridis, Scott Barcus The onedimensional, timedependent electromagnetically selfcoupled Dirac equation is solved numerically by means of the staggeredleapfrog algorithm. After the stability region of the method versus the interaction strength and the spatialgrid size over timestep ratio is established, the expectation values of several dynamic operators are evaluated as functions of time. These include the fermion and electromagnetic energies and the fermion dynamic mass, as the selfinteracting spinors are no longer masseigenfunctions. There is a characteristic, nonexponential, oscillatory dependence leading to asymptotic, timeaverages of these expectation values. In the case of the fermion mass this amounts to renormalization. The dependence of the expectation values on the spatialgrid size is evaluated in detail. [Preview Abstract] 
Sunday, April 14, 2013 2:30PM  2:42PM 
J15.00006: New Computational method for solving the timebased Dirac Equation Robert Vaselaar, Hyun Lim, JungHan Kimn, Dongming Mei Current computational methods for the Dirac equation are prone to negative behavior such as fermion doubling, instability, and poor performance for lowmass particles. These issues are usually addressed by artificial stabilizers and careful aftersimulation tuning but this may cast doubt on the physical accuracy of computational results. We show that our spacetime finite element method for the timebased Dirac equation converges to analytic solutions without artificial stabilization or aftersimulation tuning even in the lowmass regime. This method may be an important tool for simulating partially understood particles such as neutrinos where lowmass performance is essential and aftersimulation tuning is inappropriate. [Preview Abstract] 
Sunday, April 14, 2013 2:42PM  2:54PM 
J15.00007: QuarkGluon Plasma in the NDL Equation of State for Supernova Simulations J. Pocahontas Olson, Matthew Meixner, Grant Mathews, L. Nguyen, H.E. Dalhed I will discuss the effects of a QCD restored chiral symmetry and deconfined phase in the new NDL Equation of State. The transition allows for the possibility of a coexistence mixed phase using a firstorder phase transition with a Gibbs construction. I will describe the effects of temperature and pion creation on the onset density. I will also discuss the consequences of varying the QCD bag constant. The observation of a $1.97 \pm 0.04 M_\odot$ neutron star provides a stringent limit on the parameter space of a quarkgluon plasma phase in simulating supernovae collapse. The consequences of this experimental evidence on the existence and properties of a mixed phase QGP transition will be explored. [Preview Abstract] 
Sunday, April 14, 2013 2:54PM  3:06PM 
J15.00008: Center Vortices and Confinement Derar Altarawneh, Michael Engelhardt A promising picture of confinement in QCD can be obtained based on a condensate of thick vortices with fluxes in the center of the gauge group (center vortices). A number of studies of this picture have been made and specific models have been formulated to obtain a concrete realization of the vortex picture. In our model, vortices are represented by closed random lines in 2+1 dimensional spacetime. These random lines are modeled as being piecewise linear and an ensemble is generated by Monte Carlo methods. The physical space on which the vortex lines are defined is a cube with periodic boundary conditions, and I have developed the necessary algorithms which implement those boundary conditions as the vortex lines evolve across the boundaries. When two vortices become close to each other, it is possible that they connect to one another. Also the inverse process, that a vortex separates at a bottleneck, is allowed. My ensemble therefore will contain not a fixed, but a variable number of closed vortex lines. This is expected to be important for realizing the deconfining phase transition. After all processes have been implemented, I will be ready to start calculating Wilson loops and from that, the potential between quarks and antiquarks. We can study quark confinement and also ha [Preview Abstract] 

J15.00009: ABSTRACT WITHDRAWN 
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