Central Force Model Of Water Research Articles

A comparison of Coulombic interaction methods in non-equilibrium studies of heat transfer in water

We investigate the impact of the treatment of electrostatic interactions on the heat conduction of liquid water. With this purpose, we report a series of non-equilibrium molecular dynamics computer simulations of the Modified Central Force Model of water. We consider both the Ewald summation approach, which includes the full range of the electrostatic interactions, and the Wolf method, which uses a cutoff to truncate the long range contributions. It is shown that the relaxation of the temperature profiles towards the stationary state solution and the equation of state of the liquid are not affected by the treatment of the electrostatic interactions. However, the truncation of the interactions results in lower internal energy fluxes as well as lower thermal conductivities. We also find that the anomalous increase of the thermal conductivity of water with temperature is reproduced by the different methods considered in this work, showing that this physical behavior is independent of the treatment of the long range electrostatic interactions.

The ice/water interface: orientational order parameters for the basal, prism, {202̄1}, and {21̄1̄0} interfaces of ice Ih

Orientational order parameters for four crystallographically distinct ice/water interfaces are defined and measured in molecular dynamics computer simulations of the CF1 central force model of water. We find that all interfaces are broad, with consistently different orientational structures found among the four interfaces. We also define and measure a local order parameter in the interfaces related to oxygen–oxygen coordination number, and show that this measure of the interfacial width is extremely broad. Combining recent results for the translational order parameters, average density order parameters and diffusion coefficient order parameters, for this model of water, coupled with other simulations of rigid models of water, a clear picture of the structure of the ice/water interface emerges. This also reveals a possible explanation for the differential accumulation of certain solutes at the ice/water interface.

Equilibrium and nonequilibrium molecular-dynamics simulations of the central force model of water

Equilibrium and nonequilibrium molecular-dynamics simulations of the central force model of water (CFM) [Lemberg and Stillinger, J. Chem. Phys. 62, 1677 (1975)] are presented. We consider a model based on a functional form introduced in theoretical studies of associating systems employing integral equations [F. Bresme, J. Chem. Phys. 108, 4505 (1998)]. Results on thermodynamic, dynamic, dielectric, and coexistence properties are presented. The central force model shows satisfactory agreement with the experimental results in all these cases. In addition, nonequilibrium molecular-dynamics simulations show that the CFM predicts a decrease of the thermal conductivity with temperature, as observed in the experiment, but this dependence is reproduced qualitatively at temperatures characteristic of supercooled states. These results emphasize the need for further studies of the heat conduction and properties of water in these conditions. Overall the present potential should provide a basis for further theoretical and simulation studies of complex systems where water is present.

The ice/water interface: Molecular dynamics simulations of the basal, prism, {202̄1}, and {21̄1̄0} interfaces of ice Ih

The structure and dynamics of the {0001} (basal), {101̄0} (prism), {202̄1}, and {21̄1̄0} ice Ih/water interfaces have been investigated using molecular dynamics and the flexible CF1 central force model of water. The translational order profile, the average density profile, and the diffusion profile have been calculated for all four interfaces as a function of distance normal to the interface. Dynamical molecular trajectories have been used to explore the loss of translational order from within the crystal region, through the interface, and into the liquid region. The thickness of the interfaces has been determined from each order parameter and compared with results from rigid models of water and experiment. The high index faces have thinner interfacial regions than the basal and prism interfaces. All interfacial regions contain molecules that are neither ice-like nor water-like.

Solvation effects in the CF1 central force model of water: Molecular dynamics simulations

Molecular dynamics simulations are used to study the properties of water modelled by the CF1 potential. The shifted-force CF1 model of water is introduced and studied. By shifting the force and potential curves in such a way as to remove the long-range Coulomb tail, it is possible to form short-range potentials which capture a few features of the actual CF1 potentials.

Frequency and wave-vector dependent dielectric function of water: Collective modes and relaxation spectra

The longitudinal frequency and wave-vector dependent complex dielectric response function χ(k,ω)=1−1/ε(k,ω) is calculated in a broad range of k values by means of molecular dynamics computer simulation for a central force model of water. Its imaginary part, i.e., Im{ε(k,ω)}/|ε(k,ω)|2, shows two main contributions in the region of small k values: Debye-like orientational relaxation in the lower frequency part of the spectrum and a damped librational resonance at the high frequency wing. The Debye relaxation time does not follow a de Gennes-like pattern: τ(k) goes through a maximum at k≈k*≈1.7 Å−1, while the static polar structure factor S(k) peaks at k≈3 Å−1. The resonance frequency ω(k) and the decay decrement γ(k) show a dispersion law, indicative of a decaying optical-like mode, the libron. With an approximate normal mode approach, we analyze the origin of this mode on a molecular level which shows that it is due to a damped propagation of molecular orientational vibrations through the network of hydrogen bonds. At high k the decay, due to dissipation of collective into single particle motions, dominates. The static dielectric function is calculated on the basis of the response function spectra via the Kramers–Kronig relation. In the small k region ε(k) decreases from the macroscopic value ε≈80 to a value ≈15, i.e. it exhibits a Lorentzian-type behavior. This behavior is shown to be determined by higher order multipole correlation functions. In the intermediate and high k range, our results on ε(k) and χ(k) are in excellent agreement with data extracted from experimental partial pair correlation functions: ε(k) exhibits two divergence points on the k axis with a range of negative values in between where a maximum in χ(k) is found with χmax(k)≫1, indicative of overscreening. Consequences of quantum corrections to χ(k) with respect to a purely classical calculation are discussed and consequences are shown for the interaction energy between hydrated ions.

Nonlocal Dielectric Saturation in Liquid Water

The linear and nonlinear dielectric response of water to spatially varying electric fields of different wavelength and field strengths is calculated by means of molecular dynamics computer simulation for a central force model of water. We find that nonlinear effects are strongest at $k\ensuremath{\approx}3{\AA{}}^{\ensuremath{-}1}$, where the linear response function $\ensuremath{\chi}(k)\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1\ensuremath{-}1/\ensuremath{\epsilon}(k)$ has its main maximum. For small $k$ the response is linear up to field strengths of ${E}_{0}\ensuremath{\approx}1\mathrm{V}/\AA{}$. A phenomenological theory is discussed which reproduces the main nonlinear effects found in the simulation.

Structure of water and electrolyte near an electrode

Liquid water forms a structure in front of an electrode which is due to hydrogen bridge bonds between the molecules. This structure is reminiscent of ice-like clusters and determines the polarizability (differential capacitance) of the interface and the approach of ions, which have to reach an accommodation with this persisting structure. These results are derived from statistical mechanical calculations with integral equations for the central force model of water.

A variable charge central force model for water and its ionic dissociation products

The Central Force Model for water is adjusted to contain features of the Polarization Model for water and its ionic dissociation products. An ‘‘electronegativity’’ term which permits charge transfer between dissimilar moieties (protons and oxide ions in the water system) is introduced. A parameterization of this new Variable Charge Central Force Model (VCCF) is presented, and results on water and hydroxide ion are presented which support the view that some damage has occurred, and some improvement has been accomplished.

The double layer at the interface between a simple metal and an aqueous solution

We consider the interface between a simple metal, modelled as jellium, and an aqueous solution. In order to estimate the effect of the water on the distribution of electrons in jellium, we perform molecular dynamics simulations using the central force model for water. From these simulations, and from an effective pseudopotential for the electron-water interaction, we obtain the average interaction potential experienced by the metal electrons. Explicit model calculations for a mercury electrode predict a significant lowering of the electronic work function in water. The position of the effective image plane is also brought closer to the jellium edge, which results in better values for the double layer capacity.

Density and charge correlations in water

The radial distribution functions for a central force model of water, derived from a combination of computer simulation and integral equation techniques, are used to calculate the density-density and charge-charge structure factors. These quantities are compared with neutron scattering measurements of light and heavy water. From the behaviour of the density-density and charge-charge structure factors in the thermodynamic limit of small wavenumbers, the extent of density and charge fluctuations is estimated. In particular, the density fluctuations in water are found to extend to longer distances than charge fluctuates. The relations between the charge fluctuations and the behaviour of the static dielectric function as well as some dynamical properties are discussed.

Structure and properties of the CF1 central force model of water: Integral equation theory

The structure and properties of the CF1 central force model of water are studied using an integral equation theory, which supplements the hypernetted chain approximation with a bridge function. At the temperature 25 °C and density 1.000 g cm−3, both the intra- and intermolecular structure are in excellent agreement with computer simulations of the same model, and thermodynamic properties are in good agreement with experimental values. Comparison is made with the most popular rigid-molecule models of water. The predictions of this theory can be used as input into theories of inhomogeneous aqueous systems.

ChemInform Abstract: The Dissociation of Water. Analysis of the CF1 Central Force Model of Water.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Molecular dynamics study of an aqueous strontium chloride solution

A molecular dynamics simulation of a 1.1 m SrCl/sub 2/ solution was performed with an improved central force model for water at the experimental density at room temperature. The ion-water and ion-ion potentials were derived from ab initio calculations. The simulation extended over 4 ps at an average temperature of 298 K. The structural properties of the solution are discussed on the basis of radial distribution functions and the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. The dynamical properties are calculated from various autocorrelation functions. Results are presented for the influence of the ions on self-diffusion coefficients, hindered translations, librations, and internal vibrations of the water molecules.

Accurate integral equation theory for the central force model of liquid water and ionic solutions

The atom–atom pair correlation functions and thermodynamics of the central force model of water, introduced by Lemberg, Stillinger, and Rahman, have been calculated accurately by an integral equation method which incorporates two new developments. First, a rapid new scheme has been used to solve the Ornstein–Zernike equation. This scheme combines the renormalization methods of Allnatt, and Rossky and Friedman with an extension of the trigonometric basis-set solution of Labik and co-workers. Second, by adding approximate ‘‘bridge’’ functions to the hypernetted-chain (HNC) integral equation, we have obtained predictions for liquid water in which the hydrogen bond length and number are in good agreement with ‘‘exact’’ computer simulations of the same model force laws. In addition, for dilute ionic solutions, the ion–oxygen and ion–hydrogen coordination numbers display both the physically correct stoichiometry and good agreement with earlier simulations. These results represent a measurable improvement over both a previous HNC solution of the central force model and the ex-RISM integral equation solutions for the TIPS and other rigid molecule models of water.

Computer simulations of aqueous electrolyte solutions

This review concentrates mainly on the structural and dynamical properties of aqueous electrolyte solutions derived from MD simulations with the ST2 and an improved Central Force model for water. The ion-water pair potentials are either calculated by modelling the ions as Lennard-Jones spheres with an elementary charge at the center or based on ab initio calculations. The concentrations ranged from 0.55 to 2.2 molal with 200 water molecules in the basic periodic cube. The simulations extended over 10 ps.The structural properties of the solutions are discussed on the basis of radial distribution functions, the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. The dynamical properties are calculated from various autocorrelation functions. Results are presented for the influence of the ions on self-diffusion coefficients, hindered translations, librations and internal vibrations of the water molecules.

HNC solution for the central force model for liquid water

Beginning with the central force model for water introduced by Lemberg, Stillinger, and Rahman, the HNC approximation method has been used to calculate the atom–atom pair correlation functions of a state of liquid water. Although a stable and accurate solution to the HNC equation for the model is obtained using the Rossky–Dale algorithm, the structure and thermodynamics agree only crudely with the published molecular dynamics results for the same model and the same N, V, T state.

Molecular dynamics simulations of aqueous electrolyte solutions

Structural and dynamic properties of various aqueous electrolyte solutions have been calculated from MD simulations where the ST2 and Central Force model of water were employed [1]. The structural properties of the solutions are discussed on the basis of radial distribution functions, the orientation of the water molecules and their geometrical arrangement in the hydration shells of the ions. An example of the latter case in given in Fig. 1 for a MgCl 2 solution [2]. The figure shows that the oxygen atoms of the six water molecules in the first hydration shell of Mg ++ are positioned at the corners of a regular octahedron, while for Cl − an octahedral arrangement only is indicated. The four nearest neighbor water molecules around a central one show a tetrahedral structure and the distributions are sharper in the hydrogen atom directions than in the lone pair directions. Dynamic properties of the solutions such as self-diffusion and rotational diffusion coefficients, reorientation times of the dipole moment vector and residence time of the water molecules in the hydration ▪ shells of the ions are calculated from the simulation through autocorrelation functions and are compared with experimental results. The spectral densities of the hindered translational and librational motions result from Fourier transformation of the corresponding autocorrelation functions. The dynamic properties can be calculated separately for the water subsystems — hydration water of the cations, the anions and bulk water — and thus provide the possibility of understanding measured macroscopic properties of solutions on a molecular level. The self-diffusion coefficients for the three kinds of water in a LiI solution are given in Table I as an example [3]. t001 Self-diffusion Coefficients for Bulk Water (D b), Hydration Water of Li + (D +) and of I − (D − in Units of 10 −5 cm 2/s from an MD Simulation of a 2.2 Molal LiI Solution at 305°K.D O denotes pure water. i D i D iD O b 2.85 ± 0.08 0.84 + 1.33 ± 0.10 0.39 − 2.67 ± 0.10 0.78

On the orientation of water molecules in the hydration shell of the ions in a MgCl 2 solution

The orientation of the water molecules in the hydration shells of Mg 2+ and Cl − have been calculated from a molecular dynamics simulation of a 1.1 molal MgCl 2 solution with the central force model for water. The results are compared with information obtained from neutron diffraction studies with isotopic substitution.

Molecular Dynamics Simulation of an Aqueous MgCl2 Solution. Structural Results

Abstract A molecular dynamics simulation of a 1.1 molal aqueous MgCl2 solution has been performed employing the central force model for water. The effective pair potentials for ion-water have been derived from ab initio calculations. The basic box with a sidelength of 18.30 Å contained 200 water molecules, 8 anions and 4 cations, corresponding to an experimental density of 1.079 g/cm3 . The simulation extended over about 3.3 picoseconds at an average temperature of 309 K. The structure of the solution is described by radial distribution functions and the orientation of the water molecules in respect to physically meaningful directions. Values for the dielectric constant and hydration energies have been calculated. The strong influences of the twofold charged magnesium ion on the geometry of the water molecules and the structure of the hydration shell is discussed in comparison with the results of a previous simulation of a 2.2 molal NaCl solution.