S.-B. Zhu

A new flexible/polarizable water model

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 of liquid 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 compare...

Molecular‐dynamics computer simulation of an aqueous NaCl solution: Structure

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 properti...

Structure and dynamics of liquid water between plates

Using the recently developed SPC‐FP water model (simple point charge model with flexible bonds and polarization) and the molecular dynamics method, we investigate the structure and properties of liquid water between two rigid plates. In one case the plates are neutral and in the other the plates are electrically charged. In both cases substantial differences from bulk state water are found, structurally and dynamically. We observe some anomalies compared with normal liquids and attribute these to the breakage of hydrogen bonds under the influence of the solid–liquid interface. Adding an external torque enhances such breakage through the attempted alignment of the water molecules. A combination of these two contributions determines the resultant dynamical behavior of water between charged plates. The information obtained from this work should be helpful in the understanding of ‘‘hydrophobic effects’’ in aqueous solutions. The behavior of water near large polar or nonpolar molecular solutes is also revealed...

Memory kernel in liquid phase cis–trans isomerization

The memory kernel in liquid phase cis–trans isomerization is examined from the point of view of existing experimental data as well as MD simulations. In agreement with nonlinear system/bath coupling theories, when the generalized Langevin equation is written in the conventional way, we find that the ‘‘effective’’ memory kernel in this equation depends on the properties of the isomerizing particle, including the barrier height. In addition, it is found that the angular velocity distribution for the isomerizing molecule may be strongly influenced by ‘‘shape effects,’’ caused by actual molecular differences between the cis and trans configurations. This distribution may not be Maxwellian when considered over a partial angular range, e.g., the ‘‘barrier region.’’ If this is a general phenomenon, the selection of correct initial conditions in the reactive flux method would create uncertainties in the application of that method to the calculation of barrier crossing rates.

Theoretical study of memory kernel and velocity correlation function for condensed phase isomerization. I. Memory kernel

A unified memory kernel function is proposed on the basis of molecular dynamics simulations and comparison with experimental data. A frozen‐solvent‐like friction arises from the near‐neighbor solvent cage effect, while the interaction between the system and the disordered heat bath or ‘‘broken cage’’ gives rise to a white‐noise friction. On short time scales, the apparent friction is a ‘‘series addition’’ of these two limits, each solute molecule experiencing either an intact cage or a broken cage environment. For longer time scale behavior, the solvent is able to evolve, and some system molecules can experience both types of environments during the course of their own dynamics: the effective friction for these molecules then appears as a ‘‘parallel addition’’ of the same two contributions. This leads to a memory kernel having a simple exponential behavior. As the time scale for the system dynamics becomes still longer, the cage becomes indistinguishable from the heat bath, and a purely Markoffian relaxat...

Chemically stiff water: Ions, surfaces, pores, bubbles and biology

Abstract At water/air interfaces or in molecular contact with hydrophilic or hydrophobic surfaces, interfacial water separates the bulk from the adjoining phase. Water near dissolved molecules has interfacial properties, and many fundamental biological processes occur in so small a volume that all the water involved must be interfacial. In this paper, the stiffness of interfacial water is used to study its properties.

Nonequilibrium computer simulation of a salt solution

A nonequilibrium computer simulation is performed to investigate the relaxation of a realistic polar solvent near a rapidly dissociating ion pair. The time evolution of the reaction coordinate, the ultrashort time scale changes in solvation energy and solvent forces, the local density response, the heating of certain librational degrees of freedom, and the time‐dependent polarization are studied during the first 125 fs of the reaction. It is found that the relaxation behaviors in the anionic and cationic shells are very different. On average, the solvation process under study takes about 30‐40 fs to break the original cage. After another 50 fs, the solvated ion pair reforms a new metastable structure, which feeds energy back into the reacting system to break the cage further. This procedure is apparently repeated many times until dissociation is complete. The results obtained in this work provide a graphic picture of some of the features of ultrashort dynamics of ionic photodissociation reactions in a pol...

Polar molecule in a nonpolar liquid. A molecular dynamics study

Molecular dynamics techniques are used to study the structural changes in liquid carbon disulphide resulting from the introduction of a polar solute. It is found that both translational relaxation and rotational relaxation of the solvent in the shell surrounding the solute are faster than in the bulk liquid. Moreover, shifts in the intramolecular vibrational spectra and variations of the dielectric properties of the cage solvent are in evidence. The dynamical and spectroscopic behavior of the solute is also discussed. Microscopic friction experienced by the solute molecule for both translational and rotational motions is computed using the generalized stochastic theory. Information of this type is needed for constructing valid theories of ultrafast chemical reaction dynamics in condensed matter.

Kinetic energy imbalance in inhomogeneous materials

Abstract Using two types of computer molecular dynamics calculations, more evidence is presented supporting recently discovered non-Maxwellian velocity distributions of light-weight particles in potential-energy-rich heavy atom environments. The results suggest that activated processes such as nuclear tunneling and fusion could be enhanced by these unexpected velocity distributions.

Non-Maxwell distributions in equilibrated fluids. III

Abstract We examine by molecular dynamics methods the equilibrium velocity distribution of a finite sized system without using periodic boundary conditions. Extremely long runs with very short integration time steps were performed. The system is two dimensional with 196 host particles and 4 small test particles of low mass, the latter being subjected to an intense two dimensional corrugated force field. The smooth non-Maxwellian distributions obtained for the test particles show a remarkable consistency with those in papers I and II of this series.