A. Gamba

Molecular dynamics study of the interlayer water in phyllosilicates.

Abstract A preliminary molecular dynamics study of the interlayer water in montmorill o nite was performed by using a central force interaction model. The structural properties of the system, deduced from distribution functions, and the IR spectrum of water, calculated by Fourier transform of dipole moment autocorrelation function, are presented and compared with experimental data.

Theoretical study of the solvation of nitrogen. Two different approaches

Two different approaches were used for a theoretical study of the solvation of N2, with HF, H2O, NH3, CH4 as solvents. In the first approach, the contour maps of orientationally optimized interaction energy between N2 and one solvent molecule were computed by fast semiempirical methods (Extended Hückel and CNDO/2) in order to find a reliable but not too expensive calculation method for solvation models. In the case of the N2-H2O system, anab initio map was also evaluated for comparison. The second approach is based on the building up of clusters with one molecule of N2 surrounded by a number (2 to 8) of solvent molecules and finding the structure of such clusters by energy minimization. From the structures obtained it results that they are determined mainly by steric factors, so that clusters optimized by means of different methods are similar, despite the remarkable differences in the maps.

High-pressure behaviour of yugawaralite at different water content: an ab initio study

Abstract The influence of the water content in a partially occupied water site in yugawaralite on the pressure-induced structural modifications of this zeolite has been studied by means of ab initio molecular dynamics simulations. The effect of the water site occupancy has been singled out by comparing results from simulations, performed with the cell parameters determined via XRPD at rP and 8.8 GPa, on two model systems the cell stoichiometry of which differs only by the presence of a water molecule in such a site. Our results show that the global P-induced changes of the framework structure upon compression are mainly attributable to the template effect of Ca extra-framework cations, while the presence or absence of water in a partially occupied site affects the framework structure only at short range distances from the site. However, the overall water content plays a fundamental role in stabilizing/destabilizing the zeolite structure as a function of pressure.

An intermoleeular potential function for the N2−H2O system from ab initio MO calculations

An intermolecular potential function for the N2−H2O system is derived from ab initio MO calculations with a minimal basis set. An angular dependence of the functions is necessary in order to reproduce all the minima of the ab initio surface. The fitting appears satisfactory, especially for bound geometries. Some problems connected with the fitting of potential hypersurfaces are discussed.

A method for correcting empirical potentials. Application to water molecule.

Abstract A general method for correcting analytical potential functions is presented. The correction does not effect equilibrium properties but involves the dynamical behaviour of the function, e.g. , the vibrational frequencies. The method is applied to the CF model potential of water. Molecular Dynamics similations are in good agreement with experimental data for gas phase water.

Theoretical study on solvation of a neutral molecule: N2. Two different approaches

A wide investigation has been performed in view of a theoretical study of the solvation of neutral molecules. N2 as a test molecule and HF, H2O, NH3, CH4 as solvents have been chosen. In the former approach, the contour maps of interaction energy between N2 and one solvent molecule have been evaluated, with fast semiempirical methods (Extended Huckel and CNDO), and compared with each other, in order to find a reliable but not too expansive calculation method. For N2H2O interactions, a map has been computed also ab initio. The maps are shown, and qualitative and quantitative differences are discussed. The latter approach rests on building up clusters with one molecule of N2 surrounded by a number (2 to 8) of solvent molecules and finding the structure such clusters by energy minimization. From the reported structures it results that they are determined mainly by steric factors, so that clusters optimized by means of different methods are quite similar.