1,317 research outputs found
Development of Novel Density Functionals for Thermochemical Kinetics
A new density functional theory (DFT) exchange-correlation functional for the
exploration of reaction mechanisms is proposed. This new functional, denoted
BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for
transition state barriers but, unlike previous attempts at such a functional,
this improved accuracy does not come at the expense of equilibrium properties.
This makes it a general-purpose functional whose domain of applicability has
been extended to transition states, rather than a specialized functional for
kinetics. The improvement in BMK rests on the inclusion of the kinetic energy
density together with a large value of the exact exchange mixing coefficient.
For this functional, the kinetic energy density appears to correct `back' the
excess exact exchange mixing for ground-state properties, possibly simulating
variable exchange.Comment: J. Chem. Phys., in press (303431JCP, scheduled for August 15, 2004
issue); supplementary data available at
http://theochem.weizmann.ac.il/web/papers/BMK.htm
From ab initio quantum chemistry to molecular dynamics: The delicate case of hydrogen bonding in ammonia
The ammonia dimer (NH3)2 has been investigated using high--level ab initio
quantum chemistry methods and density functional theory (DFT). The structure
and energetics of important isomers is obtained to unprecedented accuracy
without resorting to experiment. The global minimum of eclipsed C_s symmetry is
characterized by a significantly bent hydrogen bond which deviates from
linearity by about 20 degrees. In addition, the so-called cyclic C_{2h}
structure is extremely close in energy on an overall flat potential energy
surface. It is demonstrated that none of the currently available (GGA,
meta--GGA, and hybrid) density functionals satisfactorily describe the
structure and relative energies of this nonlinear hydrogen bond. We present a
novel density functional, HCTH/407+, designed to describe this sort of hydrogen
bond quantitatively on the level of the dimer, contrary to e.g. the widely used
BLYP functional. This improved functional is employed in Car-Parrinello ab
initio molecular dynamics simulations of liquid ammonia to judge its
performance in describing the associated liquid. Both the HCTH/407+ and BLYP
functionals describe the properties of the liquid well as judged by analysis of
radial distribution functions, hydrogen bonding structure and dynamics,
translational diffusion, and orientational relaxation processes. It is
demonstrated that the solvation shell of the ammonia molecule in the liquid
phase is dominated by steric packing effects and not so much by directional
hydrogen bonding interactions. In addition, the propensity of ammonia molecules
to form bifurcated and multifurcated hydrogen bonds in the liquid phase is
found to be negligibly small.Comment: Journal of Chemical Physics, in press (305335JCP
Finite-size effects in tunneling between parallel quantum wires
We present theoretical calculations and experimental measurements which
reveal finite-size effects in the tunneling between two parallel quantum wires,
fabricated at the cleaved edge of a GaAs/AlGaAs bilayer heterostructure.
Observed oscillations in the differential conductance, as a function of bias
voltage and applied magnetic field, provide direct information on the shape of
the confining potential. Superimposed modulations indicate the existence of two
distinct excitation velocities, as expected from spin-charge separation.Comment: Accepted to Phys. Rev. Lett. 7/200
Influence of nano-mechanical properties on single electron tunneling: A vibrating Single-Electron Transistor
We describe single electron tunneling through molecular structures under the
influence of nano-mechanical excitations. We develop a full quantum mechanical
model, which includes charging effects and dissipation, and apply it to the
vibrating C single electron transistor experiment by Park {\em et al.}
{[Nature {\bf 407}, 57 (2000)].} We find good agreement and argue vibrations to
be essential to molecular electronic systems. We propose a mechanism to realize
negative differential conductance using local bosonic excitations.Comment: 7 pages, 6 figure
Mechanical Cooper pair transportation as a source of long distance superconducting phase coherence
Transportation of Cooper-pairs by a movable single Cooper-pair-box placed
between two remote superconductors is shown to establish coherent coupling
between them. This coupling is due to entanglement of the movable box with the
leads and is manifested in the supression of quantum fluctuations of the
relative phase of the order parameters of the leads. It can be probed by
attaching a high resistance Josephson junction between the leads and measuring
the current through this junction. The current is suppressed with increasing
temperature.Comment: 4 pages, 4 figures, RevTeX; Updated version, typos correcte
Carbon–water flux coupling under progressive drought
Water-use efficiency, defined as the ratio of carbon assimilation over evapotranspiration (ET), is a key metric to assess ecosystem functioning in response to environmental conditions. It remains unclear which factors control this ratio during periods of extended water-limitation, and current semi-empirical water-use efficiency models fail to reproduce observed ET dynamics for these periods. Here, we use dry-down events occurring at eddy-covariance flux tower sites in the FLUXNET database as natural experiments to assess if and how decreasing soil-water availability modifies water-use efficiency on ecosystem scale. We demonstrate that an attenuating soil-water availability factor in junction with a previously discovered additive radiation term is necessary to accurately predict ET flux magnitudes and dry-down lengths of these water-limited periods. In an analysis of the attenuation, 20–33% of the observed decline in ET was due to the previously unconsidered soil-water availability effect. We conclude by noting the rates of ET decline differ significantly between FLUXNET sites with tall and short vegetation types and discuss the dependency of this rate on the variability of seasonal dryness
Phonon distributions of a single bath mode coupled to a quantum dot
The properties of an unconventional, single mode phonon bath coupled to a
quantum dot, are investigated within the rotating wave approximation. The
electron current through the dot induces an out of equilibrium bath, with a
phonon distribution qualitatively different from the thermal one. In selected
transport regimes, such a distribution is characterized by a peculiar selective
population of few phonon modes and can exhibit a sub-Poissonian behavior. It is
shown that such a sub-Poissonian behavior is favored by a double occupancy of
the dot. The crossover from a unequilibrated to a conventional thermal bath is
explored, and the limitations of the rotating wave approximation are discussed.Comment: 21 Pages, 7 figures, to appear in New Journal of Physics - Focus on
Quantum Dissipation in Unconventional Environment
Thermoelectric effects in Kondo correlated quantum dots
In this Letter we study thermoelectric effects in ultra small quantum dots.
We study the behaviour of the thermopower, Peltier coefficient and thermal
conductance both in the sequencial tunneling regime and in the regime where
Kondo correlations develope. Both cases of linear response and non-equilibrium
induced by strong temperature gradients are considered. The thermopower is a
very sensitive tool to detect Kondo correlations. It changes sign both as a
function of temperature and temperature gradient. We also discuss violations of
the Wiedemann-Franz law.Comment: 7 pages; 5 figure
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