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Quantum chemical calculations are used to derive distributed polarizability models sufficiently accurate and compact to be used in classical Molecular Dynamics simulations of imidazolium-based room temperature ionic liquids. Two distributed polarizability models are fitted to reproduce the induction energy of three imidazolium cations (1,3-dimethyl-, 1-ethyl-3-methyl- and 1-butyl-3-methylimidazolium) and four anions (tetrafluoroborate, hexafluorophosphate, nitrate and thiocyanate anion) polarized by a point charge located successively on a grid of surrounding points. The first model includes charge-flow polarizabilities between first-neighbor atoms and isotropic dipolar polarizability on all atoms (except H), while the second model includes anisotropic dipolar polarizabilities on all atoms (except H). For the imidazolium cations, particular attention is given to the transferability of the distributed polarizability sets. The molecular polarizability and its anisotropy rebuilt by the distributed models are found to be in good agreement with the exact ab initio values for the three cations and twenty three additional conformers of 1-ethyl-3-methyl-, 1-butyl-3-methyl-, 1-pentyl-3-methyl-, and 1-hexyl-3-methylimidazolium cations.

Translational and rotational diffusion coefficients of liquid water have been computed from molecular dynamics simulation with a recent polarizable potential at 298, 400, and 550 K at very high pressure. At 298 K, the model reproduces the initial increase and the occurrence of a maximum for the translational and rotational diffusion coefficients when the pressure increases. At 400 and 550 K, translational and rotational diffusion coefficients are found to monotonically decrease when pressure increases in the gigapascal range, with the translational coefficient decreasing faster than the rotational one. At 400 K, such an evolution of the rotational diffusion coefficient contrasts with quasielastic neutron scattering results predicting a near independence of the rotational diffusion with a pressure increase above ≃0.5 GPa. An interpretation is proposed to explain this discrepancy. The pressure dependence of the translation–rotation coupling is analyzed. The anisotropy of rotational diffusion is investigated by computing the rotational diffusion tensor in a molecular system of axes and the reorientational correlation times of rank 1 and rank 2 of the inertia axes and of the OH bond vector. Deviation of the simulation data with respect to the predictions of the isotropic Debye model of rotational diffusion are quantified and can be used to estimate experimental rotational diffusion coefficients from experimental reorientational correlation times.

We assess the first mission of the GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) deep-sea multidisciplinary observatory for its technical capacity, performance and quality of recorded data. The functioning of the system was verified by analyzing oceanographic, seismological and geomagnetic measurements. Despite the mission’s short duration (21 days), its data demonstrated the observatory’s technological reliability and scientific value. After analyzing the oceanographic data, we found two different regimes of seawater circulation and a sharp and deepening pycnocline, linked to a down-welling phenomenon. The reliability of the magnetic and seismological measurements was evaluated by comparison with those made using on-land sensors. Such comparison of magnetic signals recorded by permanent land geomagnetic stations and GEOSTAR during a “quiet” day and one with a magnetic storm confirmed the correct functioning of the sensor and allowed us to estimate the seafloor observatory’s orientation. The magnitudes of regional seismic events recorded by our GEOSTAR seismometer agreed with those computed from land stations. GEOSTAR has thus proven itself reliable for integrating other deep-sea observation systems, such as modular observatories, arrays, and instrumented submarine cables.

OPEP is a suite of FORTRAN programs targeted at the optimal partitioning of molecular electric properties. It includes an interactive module for the construction of Cartesian grids of points, on which either the molecular electrostatic potential or the induction energy is mapped. The generation of distributed multipoles and polarizabilities is achieved using either the formalism of the normal equations of the least‐squares problem, which restates the fitting procedure in terms of simple matrix operations, or a statistical approach, which provides a pictorial description of the distributed models of multipoles and polarizabilities, thereby allowing the pinpointing of pathological cases. Molecular symmetry is accounted for by means of local atomic frames, which are generated in an automated fashion. A Tcl/Tk graphical user interface wraps the suite of programs, thereby making OPEP a user‐friendly package for building models of distributed multipoles and polarizabilities. OPEP is an open‐source suite of programs distributed free of charge under the GNU general public license (GPL) at . © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 997–1008, 2003

Abstract.: In this article, we analyze the results of a molecular dynamics simulation in aqueous solution of the N-methylazetidinone molecule, often used to model β-lactam antibiotics. The radial distribution functions (RDFs) corresponding to the most interesting atoms, in terms of reactivity, are presented. We focus our study on the effect of a polar environment on the molecule. The solvent structure around the system is compared to the structure of β-lactam-water complexes, as obtained in a previous study of reaction mechanisms for the neutral and alkaline hydrolyses of N-methylazetidinone. Two types of complexes have been considered which are related to different hydrolysis mechanisms having similar energy barriers at the rate-limiting step of the reaction path. In the first type, the β-lactam-water interaction takes place through the oxygen carbonyl atom and there is agreement between the maxima of the RDFs obtained here and the ab initio structure of the complexes previously reported. In the second type, the interaction takes place through the nitrogen atom and we do not predict a coordination layer around the β-lactam nitrogen atom. The results suggest that in aqueous solution hydrolysis of the carbonyl group is the most probable starting point for the overall hydrolysis reaction. Some discussion on the use of cluster models to represent the solvent effect is included.

The Petit Rhône Canyon (Gulf of Lions) is incised by a narrow meandering thalweg. Two current meters were moored near the bottom, one in the channel and the other on the levee. None of the measured currents is associated with the sediment transport peculiar to deep-sea fan building. This transport is mainly influenced by relatively high speeds (1 h means up to 48 cm/s; 1 day means > 30 cm/s) measured in diverse directions.

A recently developed NPT Monte Carlo method using density functional theory/molecular mechanics potentials is applied to study ion hydration. Test calculations for Na+and Cl−are in agreement with experimental and classical simulation results. In the case of Br−the DFT/MM atom-atom radial distribution function is in better agreement with the experimental bromide-oxygen distance than that obtained in the classical simulation. Both models predict a preferred linear BrH0 arrangement. Fluctuations of the bromide ion electronic cloud are illustrated through the calculation of the instantaneous dipole moment of the ion.

A hybrid NVE Molecular Dynamics simulation of liquid water is presented using a coupled Density Functional/Molecular Mechanics hamiltonian. The quantum subsystem is a single water molecule described by means of a triple-zeta quality basis set with polarization orbitals on oxygen and hydrogen atoms. Non-local exchange-correlation corrections are included self-consistently. The classical system is constituted by 128 classical TIP3P water molecules. Results are in reasonable agreement with experimental data and in particular a good description of the solute polarization is obtained. Large fluctuations of the instantaneous value of the dipole moment of the quantum molecule are predicted.

A new ab initio potential for the water dimer is presented. It is obtained from monomer properties and perturbation theory calculations (IMPT) on the dimer. The long range electrostatic interaction is described by distributed multipoles: charge, dipole and quadrupole on oxygen and charge and dipole on hydrogen. The induction is modelled using polarizabilities on oxygen up to quadrupole. The short range repulsive energy is modelled by an exponential site-site function incorporating site anisotropy. Two models have been used for the dispersion energy: accurate dispersion coefficients for the water dimer with Tang-Toennies damping functions, and a site-site model. Neither is entirely satisfactory. The geometry of the global minimum on the potential surface is in very good agreement with the experimental result. The virial coefficient agrees moderately well with the experimental determination; the discrepancies suggest that the potential is not deep enough.

Molecular dynamics simulations of the TIPS model of 1,2-dichloroethane at 300 K and a density of 1·24 g cm-3 are reported. Structural, conformational, dielectric and dynamical properties are investigated. The effects of the electrostatic interaction are ascertained by simulating a system in which this contribution has been suppressed. The role of the intramolecular motions on the dipolar correlation time is emphasized and an analysis of this quantity in terms of 'normal modes of relaxation' is presented.

AbstractThe static dielectric constant of the polarizable Stockmayer fluid is computed through Molecular Dynamics simulations where long range electrostatic interactions are computed either by reaction field or by lattice summation method (Ladd expansion). The effects on the simulated dielectric constant of the truncation of the Ladd expansion and of the cutoff distance in reaction field geometry have been explored for different values of the polarizability.