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The main static and dynamic properties of some ionic heteroclusters, involving K+, C6H6, and Ar, have been investigated. A new representation of the intermolecular potential energy, which takes into account both electrostatic and non-electrostatic contributions to the overall noncovalent interaction, was used. Dynamical calculations were performed for a microcanonical ensemble. Particular attention was paid to the opening of the isomerization and dissociation processes for K+-C6H6-Ar(n) and to the formation of some of its fragments at increasing temperatures of the cluster considered.

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The potential of mean force for the dissociation of a Ca-Cl ion pair in water is calculated from ab initio molecular dynamics simulations. The constraint-force method is employed to enhance sampling over the entire range of the reaction coordinate (Ca-Cl distance) from 2.2 to 6.5 Å. Particular attention is paid to equilibration of the system as it is found that the potential of mean force is highly sensitive to the hydration number of the Ca(2+) ion. The structure and polarization of hydration waters are examined in detail at three ion-ion separations of interest: the contact-ion position, the solvent-separated-ion position, and the transition state between them. The ab initio results are compared to the classical ones obtained using the CHARMM force field and the parameters of Dang and Smith. There are substantial differences between the polarization of hydration waters of Ca(2+) and Cl(-) ions at all distances, which indicates that an accurate description of Ca-Cl dissociation with nonpolarizable force fields may not be feasible. The ab initio results presented here for the Ca-Cl ion pair complements our earlier results for Na-Cl, and together they provide useful benchmarks for polarizable force fields under construction.

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LAGO is an extended cosmic ray observatory composed of water-Cherenkov detectors (WCD) placed throughout Latin America. It is dedicated to the study of various issues related to astrophysics, space weather and atmospheric physics at the regional scale. In this paper we present the design and implementation of the front-end electronics and the data acquisition system for readout of the WCDs of LAGO. The system consists of preamplifiers and a digital board sending data to a computer via an USB interface. The analog signals are acquired from three independent channels at a maximum rate of $ 1.2 Â 10 5 pulses per second and a sampling rate of 40 MHz. To avoid false trigger due to baseline fluctuations, we present in this work a baseline correction algorithm that makes it possible to use WCDs to study variations of the environmental radiation. A data logging software has been designed to format the received data. It also enables an easy access to the data for an off-line analysis, together with the operational conditions and environmental information. The system is currently used at different sites of LAGO.

rnQuantum chemical fully ab initio conformational calculations were performed for the weakly bound van der Waals CO. . .HZ dimer in the framework of the supermolecule approach. The counterpoise-corrected interaction energies, computed through fourth-order MBPT using the basis sets described recently by Sadlej, were in excellent agreement with results obtained with larger basis sets constructed to give accurate values for the electric moments and polarizabilities of CO and HZ. The relative stability of the configurations studied here was collinear structures > parallel structure > T-shaped structures. The collinear and parallel structures represent the most stable group of configurations. The present calculations show that they have values of D, between 5.89 and 10.66 meV. The T-shaped structures represent relatively less stable configurations with values of D, between 0.84 and 2.92 meV. 0 1995 John Wiley & Sons, Inc.between polar-polar (polar-nonpolar) molecules, of van der Waals in his equation of state for real gases and through the rise of quantum theory on the fundamental work of Heitler and London on their description of the weak attractive intermolecular dispersion forces that keep rare-gas atoms together [3]; they result in what is nowadays