Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X26: Simulations & Theory of Complex Materials |
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Sponsoring Units: DCOMP DMP Chair: Mike Norman, Argonne National Lab Room: LACC 501B |
Friday, March 25, 2005 8:00AM - 8:36AM |
X26.00001: Accurate Energies for Molecular Dynamics Simulations of Molecules and Liquids Invited Speaker: A method is presented to treat electrons within the many-body quantum Monte Carlo (QMC) approach ``on-the-fly'' throughout a molecular dynamics (MD) simulation. Our approach leverages the large (10--100) ratio of QMC electron to MD ion motion to couple the stochastic, imaginary-time electronic and real-time ionic trajectories. This continuous evolution of the QMC electrons results in highly accurate total energies for the full dynamical trajectory at a fraction of the cost of conventional, discrete sampling. We show that this can be achieved efficiently for both ground and excited states with only a modest overhead to an ab initio MD method. The accuracy of this dynamical QMC approach will be demonstrated for a variety of systems, phases, and properties, including optical gaps of hot silicon quantum dots, dissociation energy of a single water-molecule, and heat of vaporization of liquid water. We also evaluate forces on ions along the MD trajectories in QMC and compare these with forces computed by other methods. Finally, we carry out a molecular dynamics simulation completely within the QMC framework for both forces and energies. This work was performed under the auspices of the US Department of Energy by the University of California at the LLNL under contract no W-7405-Eng-48. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 9:12AM |
X26.00002: Dilute magnetic semiconductor quantum dots Invited Speaker: The realization of the technological promise of semiconductor nanoparticles requires a fundamental understanding of their nature and behavior. Recent research efforts have been focused on nanoscale semiconductors intentionally doped with magnetic impurities, i.e., quantum dots composed of dilute magnetic semiconductors (DMS's). DMS quantum dots possess interesting properties owing to spin interactions enhanced by quantum confinement. I will present results for the electronic, magnetic and optical properties of Mn-doped group IV, III-V and II-VI semiconductor quantum dots using first-principles density-functional theory with pseudopotentials and real-space algorithms. A comparison of the electronic structures with that of bulk materials will be made and the confinement effect on the intrinsic properties of these nanostructures will be discussed. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:48AM |
X26.00003: Multiscale Modeling of Small Molecules in Zeolite-4A Invited Speaker: Confinement within the nanoscale pores of a zeolite strongly modifies the behavior of small molecules such as water, ammonia, and the ammonium ion. Typical of many such interesting and important problems, realistic modeling of this phenomena requires simultaneously capturing the detailed behavior of chemical bonds and the possibility of collective dynamics occurring in a complex unit cell (672 atoms in the case of Zeolite-4A). Classical simulations alone cannot reliably model the breaking and formation of chemical bonds, while quantum methods alone are incapable of treating the extended length and time scales characteristic of complex dynamics. We report results from a hybrid approach combing first-principles electronic structure calculations, classical molecular dynamics and Monte Carlo simulations, and a recently developed model in which a small region treated with the Kohn-Sham density functional theory is embedded within a larger system represented by classical potentials. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:24AM |
X26.00004: Electronic Structure and Bonding in Complex Biomolecule Invited Speaker: For over a century vitamin B$_{12}$ and its enzyme cofactor derivates have persistently attracted research efforts for their vital biological role, unique Co-C bonding, rich red-ox chemistry, and recently their candidacies as drug delivery vehicles \textit{etc}. However, our understanding of this complex metalorganic molecule's efficient enzyme activated catalytic power is still controversial. We have for the first time calculated the electronic structure, Mulliken effective charge and bonding of a whole Vitamin B$_{12}$ molecule without any structural simplification by first- principles approaches based on density functional theory using structures determined by high resolution X-ray diffraction. A partial density of states analysis shows excellent agreement with X-ray absorption data and has been used successfully to interpret measured optical absorption spectra. Mulliken bonding analysis of B$_{12}$ and its derivatives reveal noticeable correlations between the two axial ligands which could be exploited by the enzyme to control the catalytic process. Our calculated X-ray near edge structure of B$_{12}$ and its derivates using Slater's transition state theory are also in good agreement with experiments. The same approach has been applied to other B$_{12}$ derivatives, ferrocene peptides, and recently DNA molecules. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X26.00005: A stacking-fault based microscopic model for platelets in diamond Alex Antonelli, Caetano Miranda, Ricardo Nunes We propose a new microscopic model for the $\{001\}$ planar defects in diamond commonly called platelets. This model is based on the formation of a metastable stacking fault, which can occur because of the ability of carbon to stabilize in different bonding configurations. In our model the core of the planar defect is basically a double layer of three-fold coordinated $sp^2$ carbon atoms embedded in the common $sp^3$ diamond structure. The properties of the model were determined using {\it ab initio} total energy calculations. All significant experimental signatures attributed to the platelets, namely, the lattice displacement along the $[001]$ direction, the asymmetry between the $[110]$ and the $[1\bar{1} 0]$ directions, the infrared absorption peak $B^\prime$ , and broad luminescence lines that indicate the introduction of levels in the band gap, are naturally accounted for in our model. The model is also very appealing from the point of view of kinetics, since naturally occurring shearing processes will lead to the formation of the metastable fault.\\ Authors acknowledge financial support from the Brazilian agencies FAPESP, CNPq, FAEP-UNICAMP, FAPEMIG, and Instituto do Mil\^enio em Nanoci\^encias-MCT [Preview Abstract] |
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