158 research outputs found
Double hybrid density-functional theory using the Coulomb-attenuating method
A double hybrid approximation using the Coulomb-attenuating method (CAM-DH)
is derived within range-separated density-functional perturbation theory, in
the spirit of a recent work by Cornaton {\it et al.} [Phys. Rev. A 88, 022516
(2013)]. The energy expression recovered through second order is linear in the
parameters and that control the Coulomb attenuation. The
method has been tested within the local density approximation on a small test
set consisting of rare-gas and alkaline-earth-metal dimers as well as diatomics
with single, double and triple bonds. In this context, the semi-empirical
and parameters, that were optimized for the hybrid
CAM-B3LYP functional, do not provide accurate interaction and total energies.
Using semi-local functionals with density scaling, that was neglected in this
work, may lead to different conclusions. Calibration studies on a larger test
set would be necessary at this point. This is left for future work. Finally, we
propose as a perspective an alternative CAM-DH approach that relies on the
perturbation expansion of a partially long-range interacting wavefunction. In
this case the energy is not linear anymore in and . Work is in
progress in this direction.Comment: 36 pages, 6 figure
Alternative separation of exchange and correlation energies in range-separated density-functional perturbation theory
An alternative separation of short-range exchange and correlation energies is
used in the framework of second-order range-separated density-functional
perturbation theory. This alternative separation was initially proposed by
Toulouse et al. [Theor. Chem. Acc. 114, 305 (2005)] and relies on a long-range
interacting wavefunction instead of the non-interacting Kohn-Sham one. When
second-order corrections to the density are neglected, the energy expression
reduces to a range-separated double-hybrid (RSDH) type of functional, RSDHf,
where "f" stands for "full-range integrals" as the regular full-range
interaction appears explicitly in the energy expression when expanded in
perturbation theory. In contrast to usual RSDH functionals, RSDHf describes the
coupling between long- and short-range correlations as an orbital-dependent
contribution. Calculations on the first four noble-gas dimers show that this
coupling has a significant effect on the potential energy curves in the
equilibrium region, improving the accuracy of binding energies and equilibrium
bond distances when second-order perturbation theory is appropriate.Comment: 5 figure
Use of groundwater lifetime expectancy for the performance assessment of a deep geologic waste repository: 1. Theory, illustrations, and implications
Long-term solutions for the disposal of toxic wastes usually involve
isolation of the wastes in a deep subsurface geologic environment. In the case
of spent nuclear fuel, if radionuclide leakage occurs from the engineered
barrier, the geological medium represents the ultimate barrier that is relied
upon to ensure safety. Consequently, an evaluation of radionuclide travel times
from a repository to the biosphere is critically important in a performance
assessment analysis. In this study, we develop a travel time framework based on
the concept of groundwater lifetime expectancy as a safety indicator. Lifetime
expectancy characterizes the time that radionuclides will spend in the
subsurface after their release from the repository and prior to discharging
into the biosphere. The probability density function of lifetime expectancy is
computed throughout the host rock by solving the backward-in-time solute
transport adjoint equation subject to a properly posed set of boundary
conditions. It can then be used to define optimal repository locations. The
risk associated with selected sites can be evaluated by simulating an
appropriate contaminant release history. The utility of the method is
illustrated by means of analytical and numerical examples, which focus on the
effect of fracture networks on the uncertainty of evaluated lifetime
expectancy.Comment: 11 pages, 8 figures; Water Resources Research, Vol. 44, 200
Complete analytic anharmonic hyper-Raman scattering spectra
We present the first computational treatment of the complete second-order vibrational perturbation
theory applied to hyper-Raman scattering spectroscopy. The required molecular properties are
calculated in a fully analytic manner using a recently developed program [Ringholm, Jonsson and
Ruud, J. Comp. Chem., 2014, 35, 622] that utilizes recursive routines. For some of the properties,
these calculations are the first analytic calculations of their kind at their respective levels of theory.
We apply this approach to the calculation of the hyper-Raman spectra of methane, ethane and
ethylene and compare these to available experimental data. We show that the anharmonic
corrections have a larger effect on the vibrational frequencies than on the spectral intensities, but
that the inclusion of combination and overtone bands in the anharmonic treatment can improve the
agreement with the experimental data, although the quality of available experimental data limits a
detailed comparison
Stochastic forecasts of seawater intrusion towards sustainable groundwater management: application to the Korba aquifer (Tunisia)
A stochastic study of long-term forecasts of seawater intrusion with an application to the Korba aquifer (Tunisia) is presented. Firstly, a geostatistical model of the exploitation rates was constructed, based on a multi-linear regression model combining incomplete direct data and exhaustive secondary information. Then, a new method was designed and used to construct a geostatistical model of the hydraulic conductivity field by combining lithological information and data from hydraulic tests. Secondly, the effects of the uncertainties associated with the pumping rates and the hydraulic conductivity field on the 3D density-dependent transient model were analysed separately and then jointly. The forecasts of the impacts of two different management scenarios on seawater intrusion in the year 2048 were performed by means of Monte Carlo simulations, accounting for uncertainties in the input parameters as well as possible changes of the boundary conditions. Combining primary and secondary data allowed maps of pumping rates and the hydraulic conductivity field to be constructed, despite a lack of direct data. The results of the stochastic long-term forecasts showed that, most probably, the Korba aquifer will be subject to important losses in terms of regional groundwater resource
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