1,247 research outputs found
Static dielectric response of icosahedral fullerenes from C60 to C2160 by an all electron density functional theory
The static dielectric response of C60, C180, C240, C540, C720, C960, C1500,
and C2160 fullerenes is characterized by an all-electron density-functional
method. First, the screened polarizabilities of C60, C180, C240, and C540, are
determined by the finite-field method using Gaussian basis set containing 35
basis functions per atom. In the second set of calculations, the unscreened
polarizabilities are calculated for fullerenes C60 through C2160 from the
self-consistent Kohn-Sham orbitals and eigen-values using the sum-over-states
method. The approximate screened polarizabilities, obtained by applying a
correction determined within linear response theory show excellent agreement
with the finite-field polarizabilities. The static dipole polarizability per
atom in C2160 is (4 Angstrom^3) three times larger than that in C60 (1.344
Angstrom^3). Our results reduce the uncertainty in various theoretical models
used previously to describe the dielectric response of fullerenes and show that
quantum size effects in polarizability are significantly smaller than
previously thought.Comment: RevTex, 3 figure
Electronic structure, vibrational stability, infra-red, and Raman spectra of B24N24 cages
We examine the vibrational stability of three candidate structures for the
B24N24 cage and report their infra-red (IR) and Raman spectra. The candidate
structures considered are a round cage with octahedral O symmetry, a cage with
S_4 symmetry that satisfies the isolated square rule, and a cage of S_8
symmetry, which combines the caps of the (4,4) nanotube, and contains two extra
squares and octagons. The calculations are performed within density functional
theory, at the all electron level, with large basis sets, and within the
generalized gradient approximation. The vertical ionization potential (VIP) and
static dipole polarizability are also reported. The
S_4 and S_8 cages are energetically nearly degenerate and are favored over
the O cage which has six extra octagons and squares. The IR and Raman spectra
of the three clusters show notable differences providing thereby a way to
identify and possibly synthesize the cages.Comment: (Uses Elsevier style file; To appear in Chemical Physics Letters
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
Theoretical infra-red, Raman, and Optical spectra of the B36N36 cage
The B36N36 fullerene-like cage structure was proposed as candidate structure
for the single-shell boron-nitride cages observed in electron-beam irradiation
experiment. We have performed all electron density functional calculations,
with large polarized Gaussian basis sets, on the B36N36 cage. We show that the
cage is energetically and vibrationally stable. The infra-red, Raman and
optical spectra are calculated. The predicted spectra, in combination with
experimentally measured spectra, will be useful in conclusive assignment of the
proposed B36N36 cage. The vertical and adiabatic ionization potentials as well
as static dipole polarizability are also reported.Comment: RevTex, 4 pages, 4 figures (TO appear in Physical Review A (Breif
Report)
The Hamiltonian of the V Spin System from first-principles Density-Functional Calculations
We report first-principles all-electron density-functional based studies of
the electronic structure, magnetic ordering and anisotropy for the V
molecular magnet. From these calculations, we determine a Heisenberg
Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling.
We perform direct diagonalization to determine the temperature dependence of
the susceptibility. This Hamiltonian reproduces the experimentally observed
spin =1/2 ground state and low-lying =3/2 excited state. A small
anisotropy term is necessary to account for the temperature independent part of
the magnetization curve.Comment: 4 pages in RevTeX format + 2 ps-figures, accepted by PRL Feb. 2001
(previous version was an older version of the paper
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
Effect of local Coulomb interactions on the electronic structure and exchange interactions in Mn12 magnetic molecules
We have studied the effect of local Coulomb interactions on the electronic
structure of the molecular magnet Mn12-acetate within the LDA+U approach. The
account of the on-site repulsion results in a finite energy gap and an integer
value of the molecule's magnetic moment, both quantities being in a good
agreement with the experimental results. The resulting magnetic moments and
charge states of non-equivalent manganese ions agree very well with
experiments. The calculated values of the intramolecular exchange parameters
depend on the molecule's spin configuration, differing by 25-30% between the
ferrimagnetic ground state and the completely ferromagnetic configurations. The
values of the ground-state exchange coupling parameters are in reasonable
agreement with the recent data on the magnetization jumps in megagauss magnetic
fields. Simple estimates show that the obtained exchange parameters can be
applied, at least qualitatively, to the description of the spin excitations in
Mn12-acetate.Comment: RevTeX, LaTeX2e, 4 EPS figure
Static dipole polarizability of C70 fullerene
The electronic and vibrational contributions to the static dipole
polarizability of C70 fullerene are determined using the finite-field method
within the density functional formalism. Large polarized Gaussian basis sets
augmented with diffuse functions are used and the exchange-correlation effects
are described within the Perdew-Burke-Ernzerhof generalized gradient
approximation (PBE-GGA). The calculated polarizability of C70 is 103
Angstrom^3, in excellent agreement with the experimental value of 102
Angstrom^3, and is completely determined by the electronic part, vibrational
contribution being negligible. The ratio of polarizabilities of C70 and C60 is
1.26. The comparison of polarizability calculated with only local terms (LDA)
in the PBE functional to that obtained with PBE-GGA shows that LDA is
sufficient to determine the static dipole polarizability of C70.Comment: IOP style, 1 figur
Investigation of the Jahn-Teller Transition in TiF3 using Density Functional Theory
We use first principles density functional theory to calculate electronic and
magnetic properties of TiF3 using the full potential linearized augmented plane
wave method. The LDA approximation predicts a fully saturated ferromagnetic
metal and finds degenerate energy minima for high and low symmetry structures.
The experimentally observed Jahn-Teller phase transition at Tc=370K can not be
driven by the electron-phonon interaction alone, which is usually described
accurately by LDA.
Electron correlations beyond LDA are essential to lift the degeneracy of the
singly occupied Ti t2g orbital. Although the on-site Coulomb correlations are
important, the direction of the t2g-level splitting is determined by the
dipole-dipole interactions. The LDA+U functional predicts an aniferromagnetic
insulator with an orbitally ordered ground state. The input parameters U=8.1 eV
and J=0.9 eV for the Ti 3d orbital were found by varying the total charge on
the TiF ion using the molecular NRLMOL code. We estimate the
Heisenberg exchange constant for spin-1/2 on a cubic lattice to be
approximately 24 K. The symmetry lowering energy in LDA+U is about 900 K per
TiF3 formula unit.Comment: 7 pages, 9 figures, to appear in Phys. Rev.
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