525 research outputs found
Electronic, magnetic, and vibrational properties of the molecular magnet Mn4 monomer and dimer
A new type of the single-molecule magnet [Mn_4 O_3 Cl_4 (O_2 CEt)_3(py)_3]
forms dimers. Recent magnetic hysteresis measurements on this single-molecular
magnet revealed interesting phenomena: an absence of quantum tunneling at zero
magnetic field and tunneling before magnetic field reversal. This is attributed
to a significant antiferromagnetic exchange interaction between different
monomers. To investigate this system, we calculate the electronic structure,
magnetic properties, intramolecular and intermolecular exchange interactions
using density-functional theory within the generalized-gradient approximation.
Our calculations agree with experiment. We also calculate vibrational infrared
absorption and Raman scattering intensities for the monomer which can be tested
experimentally.Comment: submitted to Journal of Physics and Chemistry of Solid
Stability of As [n=4, 8, 20, 28, 32, 36, 60] Cage Structures
We present all-electron density functional study of the geometry, electronic
structure, vibrational modes, polarizabilities as well as the infrared and
Raman spectra of fullerene-like arsenic cages. The stability of As cages
for sizes 4, 8, 20, 28, 32, 36, and 60 wherein each As atom is three-fold
coordinated is examined. We find that all the cages studied are vibrationally
stable and while all the clusters are energetically stable with respect to
isolated arsenic atoms, only As is energetically stable against
dissociation into As. We suggest that the Raman spectra might be a means
for observing the As molecule in gas phase.Comment: Uses elsart.cls (Elsevier Science), (Better pictures can be obtained
from authors); Manuscript to appear in Chemical Physics Letter
Magnetic ordering, electronic structure and magnetic anisotropy energy in the high-spin Mn single molecule magnet
We report the electronic structure and magnetic ordering of the single
molecule magnet [MnO(2,2'-biphenoxide)Br]
based on first-principles all-electron density-functional calculations. We find
that two of the ten core Mn atoms are coupled antiferromagnetically to the
remaining eight, resulting in a ferrimagnetic ground state with total spin
S=13. The calculated magnetic anisotropy barrier is found to be 9 K in good
agreement with experiment. The presence of the Br anions impact the electronic
structure and therefore the magnetic properties of the 10 Mn atoms. However,
the electric field due to the negative charges has no significant effect on the
magnetic anisotropy.Comment: 4 pages, submitted to PR
Theoretical calculations of magnetic order and anisotropy energies in molecular magnets
We present theoretical electronic structure calculations on the nature of electronic states and the magnetic coupling in the Mn12O12 free cluster and the Mn12O12(RCOO)16(H2O)4 molecular magnetic crystal. The calculations have been performed with the all-electron full-potential NRLMOL code. We find that the free Mn12O12cluster relaxes to an antiferromagneticcluster with no net moment. However, when coordinated by sixteen HCOO ligands and four H2O groups, as it is in the molecular crystal, we find that the ferrimagnetic ordering and geometrical and magnetic structure observed in the experiments is restored. Local Mn moments for the free and ligandated molecular magnets are presented and compared to experiment. We identify the occupied and unoccupied electronic states that are most responsible for the formation of the large anisotropy barrier and use a recently developed full-space and full-potential method for calculating the spin–orbit coupling interaction and anisotropy energies. Our calculated second-order anisotropy energy is in excellent agreement with experiment
Electronic structure of S-C6H5 self-assembled monolayers on Cu(111) and Au(111) substrates
We use first principles density functional theory to calculate the electronic
structure of the phenylthiolate (S-CH) self-assembled monolayer (SAM)
on Cu(111) and Au(111) substrates. We find significant lateral dispersion of
the SAM molecular states and discuss its implications for transport properties
of the molecular wire array. We calculate the two photon photoemission spectra
and the work function of the SAM on Cu(111) and compare them with the available
experimental data. Our results are used to discuss assignments of the observed
spectral data and yield predictions for new electronic states due to the
monolayer not yet accessed experimentally.Comment: 7 pages, 7 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
Theoretical polarization dependence of the two-phonon double-resonant Raman spectra of graphene
The experimental Raman spectra of graphene exhibit a few intense two-phonon
bands, which are enhanced through double-resonant scattering processes. Though
there are many theoretical papers on this topic, none of them predicts the
spectra within a single model. Here, we present results for the two-phonon
Raman spectra of graphene calculated by means of the quantum perturbation
theory. The electron and phonon dispersions, electronic lifetime,
electron-photon and electron-phonon matrix elements, are all obtained within a
density-functional-theory-based non-orthogonal tight-binding model. We study
systematically the overtone and combination two-phonon Raman bands, and, in
particular, the energy and polarization dependence of their Raman shift and
intensity. We find that the ratio of the integrated intensities for parallel
and cross polarized light for all two-phonon bands is between 0.33 and 0.42.
Our results are in good agreement with the available experimental data
5,6-dihydroxyindole-2-carboxylic acid (DHICA): a First Principles Density-Functional Study
We report first principles density functional calculations for
5,6-dihydroxyindole-2-carboxylic acid (DHICA) and several reduced forms. DHICA
and 5,6-dihydroxyindole (DHI) are believed to be the basic building blocks of
the eumelanins. Our results show that carboxylation has a significant effect on
the physical properties of the molecules. In particular, the relative
stabilities and the HOMO-LUMO gaps (calculated with the SCF method) of
the various redox forms are strongly affected. We predict that, in contrast to
DHI, the density of unpaired electrons, and hence the ESR signal, in DHICA is
negligibly small.Comment: 5 pages, 2 figure
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
Quasiparticle energies for large molecules: a tight-binding GW approach
We present a tight-binding based GW approach for the calculation of
quasiparticle energy levels in confined systems such as molecules. Key
quantities in the GW formalism like the microscopic dielectric function or the
screened Coulomb interaction are expressed in a minimal basis of spherically
averaged atomic orbitals. All necessary integrals are either precalculated or
approximated without resorting to empirical data. The method is validated
against first principles results for benzene and anthracene, where good
agreement is found for levels close to the frontier orbitals. Further, the size
dependence of the quasiparticle gap is studied for conformers of the polyacenes
() up to n = 30.Comment: 10 pages, 5 eps figures submitted to Phys. Rev.
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