32,860 research outputs found
Accurate Evaluation of Charge Asymmetry in Aqueous Solvation
Charge hydration asymmetry (CHA)--a characteristic dependence of hydration
free energy on the sign of the solute charge--quantifies the asymmetric
response of water to electric field at microscopic level. Accurate estimates of
CHA are critical for understanding hydration effects ubiquitous in chemistry
and biology. However, measuring hydration energies of charged species is
fraught with significant difficulties, which lead to unacceptably large (up to
300%) variation in the available estimates of the CHA effect. We circumvent
these difficulties by developing a framework which allows us to extract and
accurately estimate the intrinsic propensity of water to exhibit CHA from
accurate experimental hydration free energies of neutral polar molecules.
Specifically, from a set of 504 small molecules we identify two pairs that are
analogous, with respect to CHA, to the K+/F- pair--a classical probe for the
effect. We use these "CHA-conjugate" molecule pairs to quantify the intrinsic
charge-asymmetric response of water to the microscopic charge perturbations:
the asymmetry of the response is strong, ~50% of the average hydration free
energy of these molecules. The ability of widely used classical water models to
predict hydration energies of small molecules correlates with their ability to
predict CHA
Leray and LANS- modeling of turbulent mixing
Mathematical regularisation of the nonlinear terms in the Navier-Stokes
equations provides a systematic approach to deriving subgrid closures for
numerical simulations of turbulent flow. By construction, these subgrid
closures imply existence and uniqueness of strong solutions to the
corresponding modelled system of equations. We will consider the large eddy
interpretation of two such mathematical regularisation principles, i.e., Leray
and LANS regularisation. The Leray principle introduces a {\bfi
smoothed transport velocity} as part of the regularised convective
nonlinearity. The LANS principle extends the Leray formulation in a
natural way in which a {\bfi filtered Kelvin circulation theorem},
incorporating the smoothed transport velocity, is explicitly satisfied. These
regularisation principles give rise to implied subgrid closures which will be
applied in large eddy simulation of turbulent mixing. Comparison with filtered
direct numerical simulation data, and with predictions obtained from popular
dynamic eddy-viscosity modelling, shows that these mathematical regularisation
models are considerably more accurate, at a lower computational cost.Comment: 42 pages, 12 figure
Hilbert space structure of a solid state quantum computer: two-electron states of a double quantum dot artificial molecule
We study theoretically a double quantum dot hydrogen molecule in the GaAs
conduction band as the basic elementary gate for a quantum computer with the
electron spins in the dots serving as qubits. Such a two-dot system provides
the necessary two-qubit entanglement required for quantum computation. We
determine the excitation spectrum of two horizontally coupled quantum dots with
two confined electrons, and study its dependence on an external magnetic field.
In particular, we focus on the splitting of the lowest singlet and triplet
states, the double occupation probability of the lowest states, and the
relative energy scales of these states. We point out that at zero magnetic
field it is difficult to have both a vanishing double occupation probability
for a small error rate and a sizable exchange coupling for fast gating. On the
other hand, finite magnetic fields may provide finite exchange coupling for
quantum computer operations with small errors. We critically discuss the
applicability of the envelope function approach in the current scheme and also
the merits of various quantum chemical approaches in dealing with few-electron
problems in quantum dots, such as the Hartree-Fock self-consistent field
method, the molecular orbital method, the Heisenberg model, and the Hubbard
model. We also discuss a number of relevant issues in quantum dot quantum
computing in the context of our calculations, such as the required design
tolerance, spin decoherence, adiabatic transitions, magnetic field control, and
error correction.Comment: 22 2-column pages, 11 figures. Published versio
On inferring isoprene emission surface flux from atmospheric boundary layer concentration measurements
We examine the dependence of the inferred isoprene surface emission flux from atmospheric concentration on the diurnal variability of the convective boundary layer (CBL). A series of systematic numerical experiments carried out using the mixed-layer technique enabled us to study the sensitivity of isoprene fluxes to the entrainment process, the partition of surface fluxes, the horizontal advection of warm/cold air masses and subsidence. Our findings demonstrate the key role played by the evolution of boundary layer height in modulating the retrieved isoprene flux. More specifically, inaccurate values of the potential temperature lapse rate lead to changes in the dilution capacity of the CBL and as a result the isoprene flux may be overestimated or underestimated by as much as 20%. The inferred emission flux estimated in the early morning hours is highly dependent on the accurate estimation of the discontinuity of the thermodynamic values between the residual layer and the rapidly forming CBL. Uncertainties associated with the partition of the sensible and latent heat flux also yield large deviations in the calculation of the isoprene surface flux. Similar results are obtained if we neglect the influence of warm or cold advection in the development of the CBL.We show that all the above-mentioned processes are non-linear, for which reason the dynamic and chemical evolutions of the CBL must be solved simultaneously. Based on the discussion of our results, we suggest the measurements needed to correctly apply the mixed-layer technique in order to minimize the uncertainties associated with the diurnal variability of the convective boundary layer
The Likelihood of Mixed Hitting Times
We present a method for computing the likelihood of a mixed hitting-time
model that specifies durations as the first time a latent L\'evy process
crosses a heterogeneous threshold. This likelihood is not generally known in
closed form, but its Laplace transform is. Our approach to its computation
relies on numerical methods for inverting Laplace transforms that exploit
special properties of the first passage times of L\'evy processes. We use our
method to implement a maximum likelihood estimator of the mixed hitting-time
model in MATLAB. We illustrate the application of this estimator with an
analysis of Kennan's (1985) strike data.Comment: 35 page
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