Jahn-Teller instability in C6H6+ and C6H6- revisited

The benzene cation (C6H6+) has a doublet (e_{1g}) ground state in hexagonal ring (D_{6h}) geometry. Therefore a Jahn-Teller (JT) distortion will lower the energy. The present theoretical study yields a model Huckel-type Hamiltonian that includes the JT coupling of the e_{1g} electronic ground state with the two e_{2g} vibrational modes: in-plane ring-bending and C-C bond-stretching. We obtain the JT couplings from density functional theory (DFT), which gives a JT energy lowering of 970 cm^{-1} in agreement with previous quantum chemistry calculations. We find a non-adiabatic solution for vibrational spectra and predict frequencies shifts of both the benzene cation and anion, and give a reinterpretation of the available experimental data.Comment: 6 pages, 3 figure

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

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

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

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