20,242 research outputs found
Non-conformal coarse-grained potentials for water
Water is a notoriously difficult substance to model both accurately and
efficiently. Here, we focus on descriptions with a single coarse-grained
particle per molecule using the so-called Approximate Non-Conformal (ANC) and
generalized Stockmayer potentials as the starting points. They are fitted using
the radial density function and the density of the atomistic SPC/E model by
downhill simplex optimization. We compare the results with monatomic water
(mW), ELBA, as well as with direct Iterative Boltzmann Inversion (IBI) of
SPC/E. The results show that symmetrical potentials result in non-transferable
models, that is, they need to be reparametrized for new state-points. This
indicates that transferability may require more complex models. Furthermore,
the results also show that the addition of a point dipole is not sufficient to
make the potentials accurate and transferable to different temperatures (300
K-500 K) and pressures without an appropriate choice of properties as targets
during model optimization
Self-consistent calculation of particle-hole diagrams on the Matsubara frequency: FLEX approximation
We implement the numerical method of summing Green function diagrams on the
Matsubara frequency axis for the fluctuation exchange (FLEX) approximation. Our
method has previously been applied to the attractive Hubbard model for low
density. Here we apply our numerical algorithm to the Hubbard model close to
half filling (), and for , in order to study the
dynamics of one- and two-particle Green functions. For the values of the chosen
parameters we see the formation of three branches which we associate with the a
two-peak structure in the imaginary part of the self-energy. From the imaginary
part of the self-energy we conclude that our system is a Fermi liquid (for the
temperature investigated here), since Im
around the chemical potential. We have compared our fully self-consistent FLEX
solutions with a lower order approximation where the internal Green functions
are approximated by free Green functions. These two approches, i.e., the fully
selfconsistent and the non-selfconsistent ones give different results for the
parameters considered here. However, they have similar global results for small
densities.Comment: seven pages, nine figures as ps files. Accepted in Int. J. Modern
Phys. C (1997
Heat flow in the postquasistatic approximation
We apply the postquasistatic approximation to study the evolution of
spherically symmetric fluid distributions undergoing dissipation in the form of
radial heat flow. For a model which corresponds to an incompressible fluid
departing from the static equilibrium, it is not possible to go far from the
initial state after the emission of a small amount of energy. Initially
collapsing distributions of matter are not permitted. Emission of energy can be
considered as a mechanism to avoid the collapse. If the distribution collapses
initially and emits one hundredth of the initial mass only the outermost layers
evolve. For a model which corresponds to a highly compressed Fermi gas, only
the outermost shell can evolve with a shorter hydrodynamic time scale.Comment: 5 pages, 5 figure
Frequency doubling of femtosecond pulses in walk-off compensated npp
Summary form only given. N-(4-nitrophenyl)-L-prolinol (NPP) is an organic molecular crystal developped by molecular engineering, that exhibits one of the highest phase-matchable second-order susceptibilities reported so far in the near-infrared spectral range (d/sub eff//spl ap/56 pm/V). However, the large spatial and temporal walk-off existing in NPP can limit severely the usefulness of the material away from the noncritical phase-matching (ncpm) wavelength and for shorter pulses. Here we show that subpicosecond pulses can be efficiently frequency-doubled and mixed in NPP with moderate pump intensities, by employing tilted pulse techniques. These techniques make use of the large Poynting vector walk-off exhibited by NPP crystals outside the ncpm. Such techniques are based on the diffraction of the input pump wave by a grating so that each spectral component is dispersed in a different direction, thus the resulting signal is a tilted pulse.Peer ReviewedPostprint (published version
Lasso Estimation of an Interval-Valued Multiple Regression Model
A multiple interval-valued linear regression model considering all the
cross-relationships between the mids and spreads of the intervals has been
introduced recently. A least-squares estimation of the regression parameters
has been carried out by transforming a quadratic optimization problem with
inequality constraints into a linear complementary problem and using Lemke's
algorithm to solve it. Due to the irrelevance of certain cross-relationships,
an alternative estimation process, the LASSO (Least Absolut Shrinkage and
Selection Operator), is developed. A comparative study showing the differences
between the proposed estimators is provided
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