Liem X. Dang

Computer simulations of NaCl association in polarizable water

Classical molecular dynamics computer simulations have been used to investigate the thermodynamics and kinetics of sodium chloride association in polarizable water. The simulations make use of the three‐site polarizable water model of Dang [J. Chem. Phys. 97, 2659 (1992)], which accurately reproduces many bulk water properties. The model’s static dielectric constant and relaxation behavior have been calculated and found to be in reasonable agreement with experimental results. The ion–water interaction potentials have been constructed through fitting to both experimental gas‐phase binding enthalpies for small ion–water clusters and to the measured structures and solvation enthalpies of ionic solutions. Structural properties and the potential of mean force for sodium chloride in water have been calculated. In addition, Grote–Hynes theory has been used to predict dynamical features of contact ion‐pair dissociation. All of the calculated ionic solution properties have been compared with results from simulatio...

Molecular dynamics study of water clusters, liquid, and liquid–vapor interface of water with many-body potentials

The molecular dynamics computer simulation technique is used to develop a rigid, four-site polarizable model for water. The suggested model reasonably describes the important properties of water clusters, the thermodynamic and structural properties of the liquid and the liquid/vapor interface of water. The minimum energy configurations and the binding energies for these clusters are in reasonable agreement with accurate electronic structure calculations. The model predicts that the water trimer, tetramer, and pentamer have cyclic planar minimum energy structures. A prismlike structure is predicted to be lowest in energy for the water hexamer, and a cagelike structure is the second lowest in energy, with an energy of about 0.2 kcal/mol higher than the prismlike structure. The results are consistent with recent quantum Monte Carlo simulations as well as electronic structure calculations. The computed thermodynamic properties for the model, at room temperature, including the liquid density, the enthalpy of v...

The nonadditive intermolecular potential for water revised

The results of an improved version of a nonadditive intermolecular model for water that explicitly includes the nonadditive polarization energy are reported. The original polarizable water potential model (POL1), upon which the improved version is based, was developed by Caldwell, Dang, and Kollman [J. Am. Soc. Chem. 112, 9144 (1990)]. To improve the POL1 model, we developed a new set of atomic polarizabilities that reproduce the experimental molecular polarizability for water using the atom–dipole interaction model (Applequist, Carl, and Fung [J. Am. Soc. Chem. 94, 2952 (1972)]). Using the new atomic polarizabilities, we optimized the Lennard‐Jones parameters for O–O interactions to improve the model. As expected, the new model has improved the radial distribution functions and the average potential energy for liquid water as well as the density and the average total dipole moment. The model is then used to compute the binding energies of Cs+–water clusters. Without the need for three‐body forces (ion–wa...

Photoelectron spectra of the hydrated iodine anion from molecular dynamics simulations

In this paper, we present the first calculations, based on molecular dynamics techniques, of vertical electron binding energies for the ionic clusters I−(H2O)n, (n=1–15). In these studies, we employ the polarizable water model developed recently by Dang [J. Chem. Phys. 97, 2659 (1992)]. We construct the ion–water potential so that the successive binding energies for the ionic clusters, the hydration enthalpy, and the structural properties of the aqueous ionic solution agree with the results obtained from experiments. The simulated vertical electron binding energies compare well with recent data from photoelectron spectroscopy experiments by Markovich, Giniger, Levin, and Cheshnovsky [J. Chem. Phys. 95, 9416 (1991)]. Interestingly, we obtain coordination numbers of 4 to 5 for the ionic clusters, I−(H2O)n, for n≥6. This result is smaller than the coordination number, based on the energetic properties predicted by Markovich et al. Possible reasons for this discrepancy are discussed in the paper. Furthermore,...

Development of nonadditive intermolecular potentials using molecular dynamics: Solvation of Li+ and F− ions in polarizable water

Nonadditive intermolecular potentials for ion–H2O and ion–(H2O)2 complexes (ion=Li+ and F− ) were derived using molecular‐dynamics methods. The successive H2O binding energies and structural properties of Li+(H2O)n and F−(H2O)n (n=3–6) clusters, including simulations of aqueous ionic solutions, were examined using these potential parameters. The results reproduce well‐observed solvation enthalpies as well as structural properties for these ions. For n<5, water molecules distributed almost symmetrically around the Li+ ion, while water molecules preferred to cluster on the same side of the F− ion. These features are in agreement with earlier Monte Carlo, molecular dynamics, and molecular mechanics studies on similar systems. We studied the relationships between ionic clusters and bulk simulations by comparing structural properties for these simulations. In many cases, we found these properties were quite similar. These results provide valuable information in understanding the relationships between ionic clu...

Molecular dynamics simulations of aqueous ionic clusters using polarizable water

The solvation properties of a chlorine ion in small water clusters are investigated using state‐of‐the‐art statistical mechanics. The simulations employ the polarizable water model developed recently by Dang [J. Chem. Phys. 97, 2659 (1992)]. The ion–water interaction potentials are defined such that the successive binding energies for the ionic clusters, and the solvation enthalpy, bulk vertical binding energy, and structural properties of the aqueous solution agree with the best available results obtained from experiments. Simulated vertical electron binding energies of the ionic clusters Cl−(H2O)n, (n=1–6) are found to be in modest agreement with data from recent photoelectron spectroscopy experiments. Minimum energy configurations for the clusters as a function of ion polarizability are compared with the recent quantum chemical calculations of Combariza, Kestner, and Jortner [Chem. Phys. Lett. 203, 423 (1993)]. Equilibrium cluster configurations at 200 K are described in terms of surface and interior s...

Molecular dynamics simulations of CCl4–H2O liquid–liquid interface with polarizable potential models

The results from molecular dynamics simulations of the equilibrium properties of the CCl4–H2O liquid–liquid interface at room temperature are presented. The interactions between H2O–H2O, H2O–CCl4, and CCl4–CCl4 are described using the polarizable potential models developed in our laboratory. To our knowledge, this work is the first molecular dynamics simulations of the liquid–liquid interfacial equilibrium properties that explicitly includes nonadditive polarization effects. Molecular dynamics results of a 300 ps simulation following an extensive equilibration process indicate that the liquid interface is very stable, the density profile of H2O is very smooth, while that of CCl4 exhibits some oscillations. It is found that locally there is a sharp transition from one liquid phase to the other, but the overall interface is broadened by thermal fluctuations as indicated by the liquid density profiles. Calculated radial distribution functions suggest that the local structures of CCl4 and H2O remain unchanged from the bulk liquid to the interface. However, the interface does induce orientational order of H2O and CCl4 molecules. To study the polarization effects on the liquid–liquid interfacial equilibrium properties, we have calculated the total and induced dipole moments of H2O and CCl4 molecules as a function of the distance normal to the interface. The calculated dipole moments of the water molecules near the interface are close to their gas phase values, while water molecules far from the interface have dipole moments corresponding to the bulk values. This behavior can be attributed to the changes of the hydrogen bonding patterns and the orientation of water molecules near the interface. The induced dipole moments of the CCl4 molecules near the interface, on the other hand, are significantly enhanced. This is due in part to the strong local field induced by the water molecules at the interface. The calculated electric potentials using the dipole moment approach help us to analyze the orientations of water and CCl4 molecules at the interface.

The Effect of Polarizability for Understanding the Molecular Structure of Aqueous Interfaces.

A review is presented on recent progress of the application of molecular dynamics simulation methods with the inclusion of polarizability for the understanding of aqueous interfaces. Comparisons among a variety of models, including those based on density functional theory of the neat air-water interface, are given. These results are used to describe the effect of polarizability on modeling the microscopic structure of the neat air-water interface, including comparisons with recent spectroscopic studies. Also, the understanding of the contribution of polarization to the electrostatic potential across the air-water interface is elucidated. Finally, the importance of polarizability for understanding anion transfer across an organic-water interface is shown.

Comment on Mean force potential for the calcium--chloride ion pair in water'' [J. Chem. Phys. 99, 4229 (1993)]

The interionic potential of mean force (pmf) for the Ca++–Cl− ion pair in water is computed using the molecular dynamics computer simulation technique. The calculated pmf indicates a stable contact pair (CIP) and a solvent‐separated pair (SSIP) centered at 2.9 and 5.0 A with a 2.8 kcal/mol barrier to dissociation. The SSIP well is about 2.0 kcal/mol deeper than the CIP suggesting that water molecules in the first hydration shell are strongly coordinated to the Ca++ ion. Our results do not agree with the pmf reported recently by Guardia, Robinson, Padro [J. Chem. Phys. 99, 4229 (1993)]. Possible reasons for the discrepancy are discussed.

The effect of water models on the interaction of the sodium–chloride ion pair in water: Molecular dynamics simulations

We present a comparative study of the potentials of mean force (PMF) of the sodium–chloride ion pair in the simple point charge (SPC and SPC/E) water models using thermodynamic perturbation theory and the molecular dynamics technique. In agreement with previous studies, the PMF’s display two minima corresponding to the contact and solvent‐separated ion pairs. However, the contact and solvent‐separated regions are of comparable importance for the SPC water model, whereas the solvent‐separated region is favored over the contact region for the SPC/E water model.