Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D25: Optical Response of Molecules and Clusters: Theory |
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Sponsoring Units: DCOMP DMP Chair: Sudha Srinivas, Central Michigan Univ. Room: LACC 501A |
Monday, March 21, 2005 2:30PM - 2:42PM |
D25.00001: {\it Ab initio} calculations for the photoelectron spectra of transition metal clusters Shen Li, Manuel Alemany, James Chelikowsky We present results for negatively charged transition metal clusters Cu$^-_n$ and V$^-_n$ using {\it real-space} pseudopotentials constructed within the local spin density approximation. Our method assumes no explicit basis; wave functions are evaluated on a uniform grid. Only one parameter, the grid spacing, is used to control convergence of the electronic and structural properties of the cluster. To compare with experimental photoelectron spectra, we calculated the density of states (DOS) and the electron binding energies for the lowest energy structures. We found that for larger clusters (more than 5 or 6 atoms), the DOS can give reasonable agreement with experiment. However, for smaller clusters, the DOS fails to reproduce experiment. In contrast, we found that the constrained density functional method of Massobrio, Pasquarello and Car (Phys. Rev. Lett. {\bf 75}, 2104 (1995)) works very well for these smaller clusters. [Preview Abstract] |
Monday, March 21, 2005 2:42PM - 2:54PM |
D25.00002: First-Principles Simulation of a Light Harvesting Molecular Triad Tunna Baruah, Mark Pederson We investigate the properties of a recently synthesized light harvesting molecular triad containing a diaryl-porphyrin, carotenoid and a C$_{60}$ fullerene which act as a chromophore, electron donor and electron acceptor respectively [P.A. Liddell et al., J. Am. Chem. Soc. {\bf 119}, 1400 (1997)]. The triad undergoes photo-induced charge separation process resulting in a large dipole moment for the excited state. Density-functional theory based NRLMOL code is used to optimize the geometry and electronic structure. We find that the molecular states are somewhat localized on a given component of the triad and that excitations can lead to a huge dipole moment of about 180 Debye. The rate constants for the interesting metastable particle-hole states are determined from the Kohn-Sham orbitals. We use the calculated rate constants and excitation energies to estimate various figures of merit related to solar cells such as energy storage and maximum power. The results show that this molecule can be useful for solar-cell technologies and possibly for information storage applications. [Preview Abstract] |
Monday, March 21, 2005 2:54PM - 3:06PM |
D25.00003: Efficient calculation of optical linear response of large silicon clusters. Gefei Chang, Yia-Chung Chang Nanoscale silicon clusters have potential applications as light-emitting devices and bio-sensors. Ab initio calculations of the optical linear response of small-size nanoparticles have been performed via time-dependent density functional theory (TDDFT)$^1$ and by solving many-body Bethe-Salpeter equations (MBSE)$^{2,3}$. We show that the ab initio calculations can be made much more efficient when the nanocluster possess high point group symmetry and symmetrized basis functions are used. This allows us to extend the ab initio calculation to much larger Si clusters (up to a few hundred Si atoms) on a personal computer. The optical linear response of Si nanocluster (passivated with hydrogen) as a function of cluster size is examined. The effect of phosphorus doping of Si nanocluster on its optical properties is also studied.\\ \mbox{}\\ 1. O$\hat{g}$\~{u}t,S., J. R. Chelikowsky, and S. G. Louie, PRL 80, 3162(1998); Marques, M., A. Castro, and A. Rubio, J. Chem. Phys. 115, 3006(2001). 2. Rohlfing, M., and S. G. Louie, PRL 80, 3320(1998);PRB 62, 4927(2000). 3. Grossman, J. C., M. Rohlfing, L. Mitas, S. G. Louie, and M. L. Cohen,PRL 86, 472(2001). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D25.00004: Theoretical Confirmation of the Experimental Raman Spectra of the Diamondoid Molecule: Cyclohexamantane (C$_{26}$H$_{30}$) Steven L. Richardson, Tunna Baruah, Michael J. Mehl, Mark R. Pederson Diamondoids (C$_{4n+6}$H$_{4n+12}$) are rigid, cage-like hydrocarbon molecules which are superimposable upon the diamond crystal structure. For n=1, we have the simplest diamondoid, adamantane C$_{10}$H$_{16}$. While most of the diamondoids for n$\leq$3 have been synthesized in the lab, routes for preparing larger diamondoids for n$>$4 have not yet been realized. The diamondoid, cyclohexamantane (C$_{26}$H$_{30}$), has recently been isolated from distilled Gulf Coast petroleum.[1] While its structure has been confirmed through x-ray diffraction, mass spectroscopy, and $^1$H/$^{13}$C NMR spectroscopy, its vibrational Raman spectra has only been identified through an indirect comparison with the experimental Raman spectra for adamantane and diamond. We have used density-functional theory (DFT) to calculate a Raman spectra which is in excellent agreement with the experimental Raman spectra for C$_{26}$H$_{30}$, thus providing direct vibrational proof of its existence.[2] $^1$J. E. P. Dahl {\it et al.}, Angew. Chem. Int. Ed. {\bf 42,} 2040 (2003). $^2$S. L. Richardson, T. Baruah, M. J. Mehl, and M. R. Pederson, accepted for publication in Chem. Phys. Lett. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D25.00005: Calculated Properties of B$_{10}$C$_2$ Clusters L. L. Boyer, Kyungwha Park, M. R. Pederson, W. N. Mei, R. F. Sabirianov, Xiao Cheng Zeng, Luis G. Rosa, S. Balaz, P. A. Dowben The electronic structure, total energy and vibrational properties of clusters formed by removing hydrogen from B$_{10}$C$_2$H$_{12}$ are studied using density functional methods and semi-empirical models. Properties of several metastable structures, formed using density functional methods, are compared with analogous results from model calculations and available experimental data, including photoemission, optical properties, and infra-red spectroscopy. We consider our work to be a first step toward understanding the interesting properties of semiconducting films formed by removing hydrogen from films of meta- ortho- and para- carborane, the three polytypes of B$_{10}$C$_2$H$_{12}$, deposited on metal surfaces. Agreement between experiment and theory for the carborane source molecules has been established. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D25.00006: QMC calculations of the opitical gaps of Ge Nanoclusters using CPPs Jordan Vincent, Jeongnim Kim, Richard Martin Quantum Monte Carlo (QMC) calculations of the optical gaps of hydrogen-passivated Ge clusters of size 1-3 nm are presented. Although QMC methods are the most accurate methods known for interacting electrons, there are outstanding challenges in applications to materials containing heavy atoms such as Ge. The replacement of core- electrons by a pseudopotential (PP) is a primary limiting factor for the accuracy of current methods. Previous work has proposed that atoms like Ge with shallow core states can be treated using a Hartree-Fock PP plus a core-polarization potential (CPP) to account for core-valance correlation at a many-body level[1]. The goal of our work is to quantify the effect of the CPP and to predict the properties of Ge clusters in comparison to previous results[2] calculated using the time-dependent local-density approximation (TDLDA) on the same structures. [1] A. Nicklass and H. Stoll, Mol. Phys. 86, 317 (1995). [2] A. Tsolakidis and R.M. Martin, TBP. Supported by NSF DMR-03 25939 ITR. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D25.00007: QMC Calculation of the Electronic Correlations in a Fullerene Molecule Fei Lin, Jurij Smakov, Erik Sorensen, Catherine Kallin, John Berlinsky Electronic energies are calculated for a Hubbard model on a fullerene molecule ($C_{60}$, $C_{36}$ and $C_{20}$) using projector quantum Monte Carlo (QMC). Calculations are performed to accuracy high enough to determine the pair binding energy for two electrons added to neutral molecule. The method itself is checked against a variety of other quantum Monte Carlo methods as well as exact diagonalization for smaller molecules. We will also show the comparison of our results with the Hund's rule. [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D25.00008: First Principles Calculations of the Optical Properties of Hydrogen Terminated Carbon Nanoparticles Andrew Williamson, Giulia Galli, Neil Drummond, Richard Needs First-principles density-functional (DFT) and quantum Monte Carlo (QMC) calculations of the single-particle and optical gaps of hydrogen-terminated carbon nanoparticles are presented. Both diamondoid structures constructed from adamantane cages and spherical diamond structure particles are studied. The DFT calculations confirm the previous predictions of Raty et al.[1] that for carbon nanoparticles larger than 1nm quantum confinement effects disappear and the gaps drop below those of the bulk material. The QMC calculations follow the DFT trends for the size dependence of the gap, but predict optical gaps 1-2 eV larger. We illustrate that the LUMO orbital of hydrogen-terminated carbon nanoparticles is a delocalized surface state, in contrast to silicon and germanium nanoparticles, where the LUMO is core-confined. This delocalized nature of the LUMO results in a small exciton binding, a negative electron affinity, and optical gaps of larger clusters that are below the bulk gap. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [1] J.-Y. Raty, G. Galli, C. Bostedt, T.W. van Buuren, and L.J. Terminello, Phys. Rev. Lett. 90, 037401 (2003) [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D25.00009: First-principles calculation of optical excitations in azobenzene (C$_{12}$H$_{10}$N$_2$) Murilo L. Tiago, James R. Chelikowsky The photo-isomerization of azobenzene, C$_{12}$H$_{10}$N$_2$, is a long-standing theoretical problem which has been studied recently with time-dependent density-functional theory (TDDFT) and methods based on configuration interaction (CI). The central problem resides in determining the behavior of optically excited states in the molecule as its geometric configuration evolves from cis to trans-azobenzene and vice-versa. In order to get an accurate description of optical excitations in the system, we use Green's function methods based on the GW approximation and Bethe-Salpeter Equation (BSE). In this implementation, we take advantage of the finite extent of electronic wave-functions, which reduces significantly the numerical cost, without compromising accuracy. We discuss the dependence of excitation energies with respect to the CNNC dihedral angle, as the molecule undergoes isomerization. Results obtained with the BSE and TDDFT methods are compared. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D25.00010: Properties of Nitrogen-doped nanodiamond probed by first-principles. Jean-Yves Raty, Giulia Galli Nanodiamonds have been obtained as isolated clusters (in products of detonation and on some meteoritic samples) as well as aggregates in films produced by CVD. The incorporation of nanodiamonds into devices relies on the possibility of N-doping them efficiently. In this study, we report on the properties of Nitrogen-doped nanodiamonds (up to 1.5 nm in diameter) studied by ab initio molecular dynamics. Simple chemical potential calculations indicate that nitrogen incorporates into the surface. Extrapolation of our calculations for C29, C66 and C147-based clusters indicate that nitrogen atoms could incorporate substitutionally in the particle's core when the nanodiamonds diameter reaches $\sim $10nm. We discuss the energy of the donor levels of N-doped nanodiamonds together with their possible negative electron affinity, depending on surface reconstructions and nanoparticle size. Our ab-initio results point at the possibility of reaching effective electron emission from nanodiamonds. 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. JYR acknowledges support from the Belgian FNRS. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D25.00011: Theory of dielectric properties of composites based on metallic nano-particles: Kohnâ€™s Theorem approach. Krzysztof Kempa A long-wavelength electromagnetic radiation excites only the center of mass motion of electrons confined by a parabolic potential. We show, that this Kohn's Theorem applies to the case of metallic nano-particles embedded in a dielectric matrix, since each such particle can be viewed as an electron reservoir, with essentially parabolic confinement. A simple formula for the dielectric function can be derived this way, and it emphasizes the importance of the nano-particle geometry. In particular, we show that elongated, needle-like nano-particles, affect the dielectric function of the composite much stronger than the spherical nano-particles. Of importance is also the composite morphology. Predictions of this theory, generally in excellent agreement with experiment, will be presented. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D25.00012: Electronic properties of water David Prendergast, Jeffrey Grossman, Giulia Galli Recent experiments to analyze the structure of liquid water have been carried out using x-ray absorption and photo-emission. To interpret the resulting spectra requires accurate theoretical models. To this end, we investigate the electronic properties of water at ambient conditions using \emph{ab initio} density functional theory (DFT). We generate long (250 ps) classical trajectories for large supercells (up to 256 molecules), from which uncorrelated conformations of water molecules are extracted for use in DFT calculations of the electronic structure. We find that the density of occupied states of this molecular liquid is well described with 32 molecule supercells. However the description of the density of \emph{unoccupied} states (u-EDOS) is sensitive to finite size effects. Small supercell, $\Gamma$-point calculations (32 molecules) yield a spuriously isolated state above the Fermi level. Nevertheless, the more accurate u-EDOS of large supercells may be reproduced using small supercells and increased k-point sampling. Exploiting this improved understanding of the electronic properties of liquid water, we outline efficient approaches to increasing the accuracy of calculated spectra associated with optical absorption, near-edge x-ray absorption, and x-ray photo-emission. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:06PM |
D25.00013: Ab-initio simulation of Heme using GGA+U: a step toward accurate spin-state energetics Damian Scherlis, Matteo Cococcioni, P. H.-L. Sit, Nicola Marzari Enzymatic sites containing transition metals are among the most relevant biophysical systems currently studied using first-principles quantum mechanical approaches. In this context, however, the application of ab-initio methods is often severely limited as a consequence of the inability of conventional electronic structure methods - as Hartree-Fock or DFT - to provide a qualitatively correct description of the spin-state energetics of the metal center. In this work we apply the DFT based GGA+U approach to compute the low-lying states of ligated and unligated iron heme complexes. We show that this technique, in which the Hubbard- like correction is obtained in a fully ab-initio fashion using linear-response theory, is extremely useful for the treatment of organometallic compounds, in particular the heme system, where LDA, GGA, and common hybrid DFT functionals predict incorrect spin energetics. Calculations of ligand exchange thermodynamics, spin transitions, and other properties, point to GGA+U as an appealing tool to overcome the limitations entailed by the use of DFT in the description of bioinorganic complexes. Moreover, its straightforward implementation in a plane wave basis set code allows us to address systems of hundreds of atoms on commodity workstations. [Preview Abstract] |
Monday, March 21, 2005 5:06PM - 5:18PM |
D25.00014: Bone mineral: first principles study of carbonate substitutions in hydroxyapatite Roope Astala, Malcolm J. Stott Mineral components of bone and teeth are hydroxyapatite at varying crystallinities and impurity concentrations, with carbon as a CO$^{-2}_3$ ion being the most abundant. The impurities influence the biological properties of bone and a study of their electronic and structural nature will contribute to a full understanding of the biological aspects. We perform electron density functional theory calculations to investigate CO$^{-2}_3$ on different sites in periodic hydroxyapatite lattice, as well as various charge compensation mechanisms. These include substitutions on OH$^{-}$ (A-site) and PO$^{-3}_4$ (B-site), and compensation by V$_{OH}$ or V$_{Ca}$. Defect clustering and effects of impurity concentration are studied. Substitutions of two CO$^{-2}_3$ for two PO$^{-3}_4$, compensated by a V$_{Ca}$, were found to be the most favored energetically. The $a$ lattice parameter increases for A-site substitutions, while for the B-site it typically decreases. The implications on the material properties are discussed. [Preview Abstract] |
Monday, March 21, 2005 5:18PM - 5:30PM |
D25.00015: Comparative Studies of the Energetic and Properties of gamma-GeSi2N4 and gamma-SiGe2N4 in the Spinel Structure Hongzhi Yao, Lizhi Ouyang, Wai-Yim Ching To resolve the controversy on the issue about the preferred site of A and B in Si-Ge double nitride that have geometrically ternary structure form of AB$_{2}$X$_{4}$, where A and B are cations at the tetrahedral and octahedral sites respectively and X is an anion, we have calculated phonon spectrum and thermodynamic properties of~ Si-Ge spinel nitride solid solutions by first-principles density functional method using both unit and 2*2*2 supercells. Our new results showed that $\gamma $-GeSi2N4 is more stable than $\gamma $-SiGe2N4 based purely on formation energy calculation. However, the vibrational contribution shows opposite effects. The vibrational contribution to free energy is rather small at room temperature, but at 2000K, it makes a sizable contribution, of the order of 10 eV/atom, to the free energy. Therefore, the experimentally observed stable phase is likely the results of this two competing effects. We have also carried out a detailed ab-initio calculation of all independent elastic constants for Si-Ge spinel nitride systems. The results will be compared to the experimental measurements and some other calculations. [Preview Abstract] |
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