9,559 research outputs found
Stretched chemical bonds in Si6H6: A transition from ring currents to localized pi-electrons?
Motivated by solid-state studies on the cleavage force in Si, and the
consequent stretching of chemical bonds, we here study bond stretching in the,
as yet unsynthesized, free space molecule Si6H6. We address the question as to
whether substantial bond stretching (but constrained to uniform scaling on all
bonds) can result in a transition from ring current behaviour, characteristic
say of benzene at its equilibrium geometry, to localized pi-electrons on Si
atoms. Some calculations are also recorded on dissociation into 6 SiH radicals.
While the main studies have been carried out by unrestricted Hartree-Fock (HF)
theory, the influence of electron correlation has been examined using two forms
of density functional theory. Planar Si6H6 treated by HF is bound to be
unstable, not all vibrational frequencies being real. Some buckling is then
allowed, which results in real frequencies and stability. Evidence is then
provided that the non-planar structure, as the Si-Si distance is increased,
exhibits pi-electron localization in the range 1.2-1.5 times the equilibrium
distance
Extension of the composite CBS-QB3 method to singlet diradical calculations
The composite CBS-QB3 method is widely used to obtain accurate energies of
molecules and radicals although its use in the case of singlet diradicals gives
rise to some difficulties. The problem is related to the parameterized
correction this method introduces to account for spin-contamination. We report
a new term specifically designed to describe singlet diradicals separated by at
least one CH2 unit. As a test case, we have computed the formation enthalpy of
a series of diradicals that includes hydrocarbons as well as systems involving
heteroatoms (nitrogen, oxygen). The resulting CBS-QB3 energies are very close
to experiment
Timesaving Double-Grid Method for Real-Space Electronic-Structure Calculations
We present a simple and efficient technique in ab initio electronic-structure
calculation utilizing real-space double-grid with a high density of grid points
in the vicinity of nuclei. This technique promises to greatly reduce the
overhead for performing the integrals that involves non-local parts of
pseudopotentials, with keeping a high degree of accuracy. Our procedure gives
rise to no Pulay forces, unlike other real-space methods using adaptive
coordinates. Moreover, we demonstrate the potential power of the method by
calculating several properties of atoms and molecules.Comment: 4 pages, 5 figure
Nucleation of a sodium droplet on C60
We investigate theoretically the progressive coating of C60 by several sodium
atoms. Density functional calculations using a nonlocal functional are
performed for NaC60 and Na2C60 in various configurations. These data are used
to construct an empirical atomistic model in order to treat larger sizes in a
statistical and dynamical context. Fluctuating charges are incorporated to
account for charge transfer between sodium and carbon atoms. By performing
systematic global optimization in the size range 1<=n<=30, we find that Na_nC60
is homogeneously coated at small sizes, and that a growing droplet is formed
above n=>8. The separate effects of single ionization and thermalization are
also considered, as well as the changes due to a strong external electric
field. The present results are discussed in the light of various experimental
data.Comment: 17 pages, 10 figure
Spontaneous formation and stability of small GaP fullerenes
We report the spontaneous formation of a GaP fullerene cage in ab-initio
Molecular Dynamics simulations starting from a bulk fragment. A systematic
study of the geometric and electronic properties of neutral and ionized GaP
clusters suggests the stability of hetero-fullerenes formed by a compound with
zincblend bulk structure. We find that GaP fullerenes up to 28 atoms have high
symmetry, closed electronic shells, large HOMO-LUMO energy gaps and do not
dissociate when ionized. We compare our results for GaP with those obtained by
other groups for the corresponding BN clusters.Comment: To appear on PRL, 4 pages, 1 figure, Late
Electronic structure of crystalline binary and ternary Cd-Te-O compounds
The electronic structure of crystalline CdTe, CdO, -TeO,
CdTeO and CdTeO is studied by means of first principles
calculations. The band structure, total and partial density of states, and
charge densities are presented. For -TeO and CdTeO, Density
Functional Theory within the Local Density Approximation (LDA) correctly
describes the insulating character of these compounds. In the first four
compounds, LDA underestimates the optical bandgap by roughly 1 eV. Based on
this trend, we predict an optical bandgap of 1.7 eV for CdTeO. This
material shows an isolated conduction band with a low effective mass, thus
explaining its semiconducting character observed recently. In all these oxides,
the top valence bands are formed mainly from the O 2p electrons. On the other
hand, the binding energy of the Cd 4d band, relative to the valence band
maximum, in the ternary compounds is smaller than in CdTe and CdO.Comment: 13 pages, 15 figures, 2 tables. Accepted in Phys Rev
Electronic Properties of Ultra-Thin Aluminum Nanowires
We have carried out first principles electronic structure and total energy
calculations for a series of ultrathin aluminum nanowires, based on structures
obtained by relaxing the model wires of Gulseren et al. The number of
conducting channels is followed as the wires radius is increased. The results
suggest that pentagonal wires should be detectable, as the only ones who can
yield a channel number between 8 and 10.Comment: 9 pages + 3 figures, to appear on Surface Scienc
In-depth exploration of the photophysics of a trinuclear palladium complex
Contains fulltext :
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Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)
Conformational energies of n-butane, n-pentane, and n-hexane have been
calculated at the CCSD(T) level and at or near the basis set limit.
Post-CCSD(T) contribution were considered and found to be unimportant. The data
thus obtained were used to assess the performance of a variety of density
functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP,
especially with a small Grimme-type empirical dispersion correction, are
capable of rendering conformational energies of CCSD(T) quality. These were
then used as a `secondary standard' for a larger sample of alkanes, including
isopentane and the branched hexanes as well as key isomers of heptane and
octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to
overestimate conformer energies without dispersion correction, while the M06
family severely underestimates GG interaction energies. Grimme-type dispersion
corrections for these overcorrect and lead to qualitatively wrong conformer
orderings. All of these functionals also exhibit deficiencies in the conformer
geometries, particularly the backbone torsion angles. The PW6B95 and, to a
lesser extent, BMK functionals are relatively free of these deficiencies.
Performance of these methods is further investigated to derive conformer
ensemble corrections to the enthalpy function, , and the Gibbs
energy function, , of these alkanes. While
is only moderately sensitive to the level of theory, exhibits more pronounced sensitivity. Once again, double hybrids
acquit themselves very well.Comment: J. Phys. Chem. A, revised [Walter Thiel festschrift
A Multiscale Approach to Determination of Thermal Properties and Changes in Free Energy: Application to Reconstruction of Dislocations in Silicon
We introduce an approach to exploit the existence of multiple levels of
description of a physical system to radically accelerate the determination of
thermodynamic quantities. We first give a proof of principle of the method
using two empirical interatomic potential functions. We then apply the
technique to feed information from an interatomic potential into otherwise
inaccessible quantum mechanical tight-binding calculations of the
reconstruction of partial dislocations in silicon at finite temperature. With
this approach, comprehensive ab initio studies at finite temperature will now
be possible.Comment: 5 pages, 3 figure
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