First-principle density-functional calculation of the Raman spectra of BEDT-TTF

We present a first-principles density-functional calculation for the Raman spectra of a neutral BEDT-TTF molecule. Our results are in excellent agreement with experimental results. We show that a planar structure is not a stable state of a neutral BEDT-TTF molecule. We consider three possible conformations and discuss their relation to disorder in these systems.Comment: 3 pages, 2 figures, submitted to the proceedings of ISCOM 200

Density-functional-based predictions of Raman and IR spectra for small Si clusters

We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems

Membrane solitons in eight-dimensional hyper-Kaehler backgrounds

We derive the BPS equations satisfied by lump solitons in $(2+1)$-dimensional sigma models with toric 8-dimensional hyper-K\"ahler (${HK}_8$) target spaces and check they preserve 1/2 of the supersymmetry. We show how these solitons are realised in M theory as M2-branes wrapping holomorphic 2-cycles in the \bE^{1,2}\times {HK}_8 background. Using the $\kappa$-symmetry of a probe M2-brane in this background we determine the supersymmetry they preserve, and note that there is a discrepancy in the fraction of supersymmetry preserved by these solitons as viewed from the low energy effective sigma model description of the M2-brane dynamics or the full M theory. Toric ${HK}_8$ manifolds are dual to a Hanany-Witten setup of D3-branes suspended between 5-branes. In this picture the lumps correspond to vortices of the three dimensional ${\mathcal N}=3$ or ${\mathcal N}=4$ theory.Comment: 12+1 pages. LaTex. v2: Typos corrected and references adde

Predicted Infrared and Raman Spectra for Neutral Ti_8C_12 Isomers

Using a density-functional based algorithm, the full IR and Raman spectra are calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically unstable. The calculated properties of the relaxed structure having C3v symmetry are found to be in excellent agreement with experimental gas phase infrared results, ionization potential and electron affinity measurements. Consequently, the results presented may be used as a reference for further experimental characterization using vibrational spectroscopy.Comment: 6 pages, 5 figures. Physical Review A, 2002 (in press

Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors

We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.Comment: 5 pages, four figures. Supplementary material at http://www.ruf.rice.edu/~natelson/publications.htm

Molecular structures and vibrations of neutral and anionic CuOx (x = 1-3,6) clusters

We report equilibrium geometric structures of CuO2, CuO3, CuO6, and CuO clusters obtained by an all-electron linear combination of atomic orbitals scheme within the density-functional theory with generalized gradient approximation to describe the exchange-correlation effects. The vibrational stability of all clusters is examined on the basis of the vibrational frequencies. A structure with Cs symmetry is found to be the lowest-energy structure for CuO2, while a -shaped structure with C2v symmetry is the most stable structure for CuO3. For the larger CuO6 and CuO clusters, several competitive structures exist with structures containing ozonide units being higher in energy than those with O2 units. The infrared and Raman spectra are calculated for the stable optimal geometries. ~Comment: Uses Revtex4, (Better quality figures can be obtained from authors

Transition dipole strength of eumelanin

We report the transition dipole strength of eumelanin (the principal human photoprotective pigment) in the ultraviolet and visible. We have used both theoretical (density functional) and experimental methods to show that eumelanin is not an unusually strong absorber amongst organic chromophores. This is somewhat surprising given its role as a photoprotectant, and suggests that the dark coloring in vivo (and in vitro) of the eumelanin pigment is a concentration effect. Furthermore, by observing the polymerization of a principle precursor (5,6-dihydroxyindole-2-carboxylic acid) into the full pigment, we observe that eumelanin exhibits a small amount (similar to 20%) of hyperchromism (i.e., the reaction process enhances the light absorption ability of the resultant macromolecule relative to its monomeric precursor). These results have significant implications for our understanding of the photophysics of these important functional biomolecules. In particular, they appear to be consistent with the recently proposed chemical disorder secondary structure model of eumelanins

Variational Hilbert space truncation approach to quantum Heisenberg antiferromagnets on frustrated clusters

We study the spin-$\frac{1}{2}$ Heisenberg antiferromagnet on a series of finite-size clusters with features inspired by the fullerenes. Frustration due to the presence of pentagonal rings makes such structures challenging in the context of quantum Monte-Carlo methods. We use an exact diagonalization approach combined with a truncation method in which only the most important basis states of the Hilbert space are retained. We describe an efficient variational method for finding an optimal truncation of a given size which minimizes the error in the ground state energy. Ground state energies and spin-spin correlations are obtained for clusters with up to thirty-two sites without the need to restrict the symmetry of the structures. The results are compared to full-space calculations and to unfrustrated structures based on the honeycomb lattice.Comment: 22 pages and 12 Postscript figure

A critical assessment of the Self-Interaction Corrected Local Density Functional method and its algorithmic implementation

We calculate the electronic structure of several atoms and small molecules by direct minimization of the Self-Interaction Corrected Local Density Approximation (SIC-LDA) functional. To do this we first derive an expression for the gradient of this functional under the constraint that the orbitals be orthogonal and show that previously given expressions do not correctly incorporate this constraint. In our atomic calculations the SIC-LDA yields total energies, ionization energies and charge densities that are superior to results obtained with the Local Density Approximation (LDA). However, for molecules SIC-LDA gives bond lengths and reaction energies that are inferior to those obtained from LDA. The nonlocal BLYP functional, which we include as a representative GGA functional, outperforms both LDA and SIC-LDA for all ground state properties we considered.Comment: 14 pages, 5 figure