449 research outputs found
SAFT-γ force field for the simulation of molecular fluids: 4. A single-site coarse-grained model of water applicable over a wide temperature range
In this work, we develop coarse-grained (CG) force fields for water, where the effective CG intermolecular interactions between particles are estimated from an accurate description of the macroscopic experimental vapour-liquid equilibria data by means of a molecular-based equation of state. The statistical associating fluid theory for Mie (generalised Lennard-Jones) potentials of variable range (SAFT-VR Mie) is used to parameterise spherically symmetrical (isotropic) force fields for water. The resulting SAFT-γ CG models are based on the Mie (8-6) form with size and energy parameters that are temperature dependent; the latter dependence is a consequence of the angle averaging of the directional polar interactions present in water. At the simplest level of CG where a water molecule is represented as a single bead, it is well known that an isotropic potential cannot be used to accurately reproduce all of the thermodynamic properties of water simultaneously. In order to address this deficiency, we propose two CG potential models of water based on a faithful description of different target properties over a wide range of temperatures: our CGW1-vle model is parameterised to match the saturated-liquid density and vapour pressure; our other CGW1-ift model is parameterised to match the saturated-liquid density and vapour-liquid interfacial tension. A higher level of CG corresponding to two water molecules per CG bead is also considered: the corresponding CGW2-bio model is developed to reproduce the saturated-liquid density and vapour-liquid interfacial tension in the physiological temperature range, and is particularly suitable for the large-scale simulation of bio-molecular systems. A critical comparison of the phase equilibrium and transport properties of the proposed force fields is made with the more traditional atomistic models
Testing Gravitation in the Solar System with Radio Science experiments
The laws of gravitation have been tested for a long time with steadily
improving precision, leading at some moment of time to paradigmatic evolutions.
Pursuing this continual effort is of great importance for science. In this
communication, we focus on Solar System tests of gravity and more precisely on
possible tests that can be performed with radio science observations (Range and
Doppler). After briefly reviewing the current tests of gravitation at Solar
System scales, we give motivations to continue such experiments. In order to
obtain signature and estimate the amplitude of anomalous signals that could
show up in radio science observables because of modified gravitational laws, we
developed a new software that simulates Range/Doppler signals. We present this
new tool that simulates radio science observables directly from the space-time
metric. We apply this tool to the Cassini mission during its cruise from
Jupiter to Saturn and derive constraints on the parameters entering alternative
theories of gravity beyond the standard Parametrized Post Newtonian theory.Comment: proceedings of SF2A 2011 - minor changes (typos corrected -
references updated
SAFT-gamma Force Field for the Simulation of Molecular Fluids. 1. A Single-Site Coarse Grained Model of Carbon Dioxide
General post-Minkowskian expansion of time transfer functions
Modeling most of the tests of general relativity requires to know the
function relating light travel time to the coordinate time of reception and to
the spatial coordinates of the emitter and the receiver. We call such a
function the reception time transfer function. Of course, an emission time
transfer function may as well be considered. We present here a recursive
procedure enabling to expand each time transfer function into a perturbative
series of ascending powers of the Newtonian gravitational constant (general
post-Minkowskian expansion). Our method is self-sufficient, in the sense that
neither the integration of null geodesic equations nor the determination of
Synge's world function are necessary. To illustrate the method, the time
transfer function of a three-parameter family of static, spherically symmetric
metrics is derived within the post-linear approximation.Comment: 10 pages. Minor modifications. Accepted in Classical and Quantum
Gravit
Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments
A generalization of the recent version of the statistical associating fluid theory for variable range Mie potentials [Lafitte et al., J. Chem. Phys. 139, 154504 (2013)] is formulated within the framework of a group contribution approach (SAFT-γ Mie). Molecules are represented as comprising distinct functional (chemical) groups based on a fused heteronuclear molecular model, where the interactions between segments are described with the Mie (generalized Lennard-Jonesium) potential of variable attractive and repulsive range. A key feature of the new theory is the accurate description of the monomeric group-group interactions by application of a high-temperature perturbation expansion up to third order. The capabilities of the SAFT-γ Mie approach are exemplified by studying the thermodynamic properties of two chemical families, the n-alkanes and the n-alkyl esters, by developing parameters for the methyl, methylene, and carboxylate functional groups (CH3, CH2, and COO). The approach is shown to describe accurately the fluid-phase behavior of the compounds considered with absolute average deviations of 1.20% and 0.42% for the vapor pressure and saturated liquid density, respectively, which represents a clear improvement over other existing SAFT-based group contribution approaches. The use of Mie potentials to describe the group-group interaction is shown to allow accurate simultaneous descriptions of the fluid-phase behavior and second-order thermodynamic derivative properties of the pure fluids based on a single set of group parameters. Furthermore, the application of the perturbation expansion to third order for the description of the reference monomeric fluid improves the predictions of the theory for the fluid-phase behavior of pure components in the near-critical region. The predictive capabilities of the approach stem from its formulation within a group-contribution formalism: predictions of the fluid-phase behavior and thermodynamic derivative properties of compounds not included in the development of group parameters are demonstrated. The performance of the theory is also critically assessed with predictions of the fluid-phase behavior (vapor-liquid and liquid-liquid equilibria) and excess thermodynamic properties of a variety of binary mixtures, including polymer solutions, where very good agreement with the experimental data is seen, without the need for adjustable mixture parameters
Radioscience simulations in General Relativity and in alternative theories of gravity
In this communication, we focus on the possibility to test GR with
radioscience experiments. We present a new software that in a first step
simulates the Range/Doppler signals directly from the space time metric (thus
in GR and in alternative theories of gravity). In a second step, a
least-squares fit of the involved parameters is performed in GR. This software
allows one to get the order of magnitude and the signature of the modifications
induced by an alternative theory of gravity on radioscience signals. As
examples, we present some simulations for the Cassini mission in
Post-Einsteinian gravity and with the MOND External Field Effect.Comment: 4 pages; Proceedings of "Les Rencontres de Moriond 2011 - Gravitation
session
INPOP08, a 4-D planetary ephemeris: From asteroid and time-scale computations to ESA Mars Express and Venus Express contributions
The latest version of the planetary ephemerides developed at the Paris
Observatory and at the Besancon Observatory is presented here. INPOP08 is a
4-dimension ephemeris since it provides to users positions and velocities of
planets and the relation between TT and TDB. Investigations leading to improve
the modeling of asteroids are described as well as the new sets of observations
used for the fit of INPOP08. New observations provided by the European Space
Agency (ESA) deduced from the tracking of the Mars Express (MEX) and Venus
Express (VEX) missions are presented as well as the normal point deduced from
the Cassini mission. We show the huge impact brought by these observations in
the fit of INPOP08, especially in terms of Venus, Saturn and Earth-Moon
barycenter orbits.Comment: 14 pages. submitted to A&A. accepted in A&
Normal transport properties for a classical particle coupled to a non-Ohmic bath
We study the Hamiltonian motion of an ensemble of unconfined classical
particles driven by an external field F through a translationally-invariant,
thermal array of monochromatic Einstein oscillators. The system does not
sustain a stationary state, because the oscillators cannot effectively absorb
the energy of high speed particles. We nonetheless show that the system has at
all positive temperatures a well-defined low-field mobility over macroscopic
time scales of order exp(-c/F). The mobility is independent of F at low fields,
and related to the zero-field diffusion constant D through the Einstein
relation. The system therefore exhibits normal transport even though the bath
obviously has a discrete frequency spectrum (it is simply monochromatic) and is
therefore highly non-Ohmic. Such features are usually associated with anomalous
transport properties
- …