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A classical molecular-dynamics simulation of a 1.791 molal aqueous NaCl solution is performed using a flexible/polarizable five-site water model. Through an investigation of the ion-water pair-correlation functions and the relative orientation of the waters of hydration, we are able to study the solvation structure in this moderately concentrated salt solution. Under perturbations from the solvated ions, there appears a considerable reorganization of the water molecules. Some of the original intermolecular hydrogen bonding structure is broken down, as bonding with the neighboring cations and anions takes place. Also considered in this paper are the energetics of hydration, and the effect of ionic perturbations on properties such as the water intramolecular structure and vibrations, and the solution phase dielectric constant. An important conclusion from this work is that, in the NaCl solution studied here, perturbations on the water structure by the ions affect mainly intermolecular orientational properties. Although they may be large, these orientational effects are not sensitively detected by many experimental techniques, such as x-ray diffraction or vibrational spectroscopy.
Both geometrical flexibility and instantaneously responsive electrical polarization are incorporated into a newly developed 5-site water model that includes one oxygen atom, two partially shielded protons, and two negative charges representing lone pairs. The charges are diffusively distributed. Their values are variable in accordance with the local field. The intramolecular potential function used is the one recently developed by Dang and Pettitt [J. Phys. Chem. 91,3349 (1987)] for a free water molecule. In order to strengthen the angular dependence of the intermolecular dimer potential, a short-range Morse-type interaction is introduced to represent specific hydrogen bonding interactions. With this model we carry out a classical constant volume molecular dynamics simulation ofliquid water at mass density 0.997 g/cm3 and room temperature 298 K. Results for the liquid structure, thermodynamic properties, transport dynamics, dielectric features, and spectroscopic characteristics are presented and compared with the experimental data and other relevant computer simulations. These comparisons show a significant improvement over the 3-site flexible/polarizable model developed earlier at Texas Tech. Though about four times computationally more intensive, the new model is still simple enough to be applied to studies of liquid water in the presence of various types oflocal perturbations, where electrical fields and orientational effects specifically require geometric flexibility and electrical polarization. J. Chern. Phys. 95 (4).
A new class of potential suitable for modeling the adsorption of water on different metal sites is described. The new potentials are simple in form and convenient for use in computer simulations. In their real space form they comprise three parts: A pairwise sum of spatially anisotropic 12-6 potentials, a pairwise sum of isotropic short range potentials, and an image potential. Two modifications of the potential are developed. In the first, the anisotropic potential acts only on the oxygen atom and not on the protons. In the second, the potential acts on all the atoms of the water molecule. In practical calculations it is convenient to transform the potential to a reciprocal space form in the manner described by Steele [Surf. Sci. 36, 317 (1973)]. Adsorption of water at top, bridge, and hollow sites on (100), (110), and (111) surfaces of Pt, Ni, Cu, and Al were studied using two fitting parameters and the results compared with previous theoretical calculations.
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