Polarizable Water Model Research Articles

Electronic spectra in bulk water and at the water liquid/vapor interface.: Effect of solvent and solute polarizabilities

The role of solvent and solute many-body polarizabilities in influencing the electronic absorption line shapes for a model dipolar solute at the water liquid/vapor interface is investigated using molecular dynamics computer simulations. Several choices for the charge distribution in the solute excited state and for the solute atomic polarizability are considered. A comparison is made between the results using both polarizable and non-polarizable water models. Solvent polarizability is found to have a more substantial effect on the peak absorption spectrum than solute polarizability, especially for transitions to more polar excited states, and somewhat less so at the interface than in the bulk.

Structure and Thermodynamics of H+(H2O)n (n = 9, 21, 40) Clusters between 0 and 300 K. A Monte Carlo Study

We present a Monte Carlo study of protonated water clusters, H+(H2O)n, with size n = 9, 21, and 40, and neutral clusters, (H2O)20, in the temperature range 0 to 300 K. We study the structural differences between the solidlike and liquidlike phases, using an empirical polarizable water model. The transition between these two phases is particularly distinct in H+(H2O)21, which attains the dodecahedral cage configuration at temperatures up to 150 K, a structure that does not survive above 170 K. The results support the idea that the “magic number” behavior of H+(H2O)21 is restricted to temperatures below the melting point. We estimate the melting temperatures for H+(H2O)9, H+(H2O)21, and (H2O)20 as predicted by the model to be 130, 160, and 160 K, respectively. The melting process in the protonated clusters thus appears to be governed mainly by water−water interactions.

Electron tunneling through water layers: Effect of layer structure and thickness

The effect of thickness and molecular structure on the probability of electron tunneling through water layers is investigated using a recently developed method. Water configurations of 1–4 layers are prepared between two parallel slabs of the Pt(100) surface, using equilibrium molecular dynamics and the polarizable simple point charge water model. Electron tunneling probabilities through the different water layers are computed as functions of energy using the absorbing boundary conditions Green function method and employing either an effective two-body water–electron interaction or a many-body polarizable water–electron potential. As long as the electron incident energy is below the barrier and far from a resonance state, the tunneling probabilities can be reasonably fitted to a one-dimensional rectangular-barrier model. However, near and over-barrier transmission probabilities cannot be reasonably described using a one-dimensional model, and the three-dimensional discrete structure of the water plays an important role. In all systems, the many-body electronic polarizability of the water significantly affects the transmission probability. The role played by the first adsorbed water layer is also discussed.

Electron Tunneling through Water Layers: Effect of Polarizability

In a recent work (Mosyak, A.; et al. J. Chem. Phys. 1996, 104, 1549), we investigated numerically electron tunneling through water layers confined between two solid walls. In the present paper, the effect of some of our model assumptions and parameters on the tunneling behavior is studied. In particular, we focus on the role played by the water electronic polarizability. We find that the tunneling behavior computed with water configurations prepared with a polarizable SPC water model is very similar to that obtained with configurations prepared using the nonpolarizable RWK2-M water model used previously, provided that the same electron−water pseudopotential is invoked. On the other hand, including the self-consistent many-body potential associated with the water electronic polarizability in the model for the electron−water interaction has a profound effect on the tunneling behavior, making the tunneling probability ∼2 orders of magnitude larger than calculated with the nonpolarizable model. This rectifies...

Dielectric constant of polarizable water at elevated temperatures

The dielectric constant of a polarizable water model is estimated for several thermodynamic states at elevated temperatures and reduced densities relative to ambient conditions using molecular dynamics simulations. An extended Lagrangian formulation of induced polarization is used in these simulations in conjunction with thermostat variables. This approach to the simulation of induced polarization is found to require about one-half the computational effort needed to determine directly the adiabatic values for the induced dipole moments. As the temperature increases and the density decreases, the difference between the model and experimental values of the dielectric constant is positive and significantly larger than the uncertainty in the model values.

Molecular dynamics simulations of water/metal and water/vacuum interfaces with a polarizable water model

We report results of a comparative computer simulation study of polarizable and non-polarizable water near realistic metal surfaces. The simulations indicate that the explicit treatment of molecular polarizability does not significantly alter the interfacial structure.

Critical Study of Fluoride−Water Interactions

A new parametrization of the fluoride−water interaction within a polarizable water model is presented. Because of the absence of accurate experimental data for the enthalpy of formation of the F-(H2O) cluster, the results of ab-initio calculations were used to parametrize the ion−water interaction. The ab-initio results suggest that this interaction is 10% stronger than what was previously thought. The accuracy of the present parametrization was evaluated by comparing the model potential with the ab-initio results along the minimum energy profile for the fluoride−water interaction for various F−O separations. The energetic and structural properties of the clusters F-(H2O)n, n = 1−10, as well as of aqueous fluoride solution are studied using molecular dynamics simulation techniques. The stronger ion−water interaction results in the appearance of interior states (configurations in which the ion is “solvated” by water molecules) for finite clusters with six or more water molecules. The results of the aqueous...

Theoretical Study of H+Translocation along a Model Proton Wire

The mechanism of proton translocation along linear hydrogen-bonded water chains is investigated. Classical and discretized Feynman path integral molecular dynamics simulations are performed on protonated linear chains of 4, 5, and 9 water molecules. The dissociable and polarizable water model PM6 of Stillinger and co-workers is used to represent the potential energy surface of the systems. The simulations show that quantum and thermal effects are both important because the height of the barriers opposing proton transfer are strongly coupled to the configuration of the chain, which is, in turn, affected by the presence of an excess proton. For characterization of the quantum effects, the energy levels of the hydrogen nucleus located at the center of a protonated tetrameric water chain are calculated by solving the Schroedinger equation for an ensemble of configurations which were generated with path integral simulations. Analysis shows that the first excitation energies are significantly larger than the thermal energy kBT and that quantum effects are dominated by the zero-point energy of the proton. The quantum correlations between the different proton nuclei are found to be negligibly small, suggesting that an effective one-particle description could be valid. Potential of mean force surfaces for proton motion in relation to the donor−acceptor separation are calculated with classical and path integral simulations for tetrameric and pentameric water chains. The mechanism for long-range proton transfer is illustrated with a simulation of a hydrogen-bonded chain of nine water molecules. During the simulation, cooperative fluctuations which modulate the asymmetry of the chain enable the spontaneous translocation of protons over half of the length of the chain.

Effects of Polarizability on the Hydration of the Chloride Ion

Polarizable and nonpolarizable potential models for both water and chloride are used to address the issue of surface vs interior solvation of the chloride ion in Cl(H2O)n- clusters, for n up to 255. We find that, even for the largest clusters, simulations with polarizable water models show that the chloride ion is preferentially solvated near the surface of the cluster. This behavior is not observed with a nonpolarizable model. The many-body effects are not directly responsible for this solvation behavior; polarizability appears to be important primarily for its role in facilitating a larger average dipole moment on the water model. Polarizability on the chloride ion is not found to have a substantial effect on the structure of the clusters.

Extended Lagrangian formalism applied to temperature control and electronic polarization effects in molecular dynamics simulations

This paper describes the implementation of the extended Lagrangian method to control the temperature, and to compute electronic polarization effects in molecular dynamics simulations of complex systems. Temperature control is achieved by introducing an extra variable coupled to the velocities, whereas induced dipoles are represented as Drude oscillators that respond to the electric field produced by the surrounding particles. Computing the contribution from the induced dipoles therefore requires no iteration or matrix inversion procedures and is as fast as evaluating classical non-bonded interactions. It is shown how a judicious choice of the integration algorithm makes the implementation of both procedures straightforward. The application in a molecular dynamics procedure, which integrates both methods, is illustrated in the constant-temperature simulations of pure water and methane-water mixtures, in which the solvent is represented by the mean-field SPC or by the polarizable PSPC water models.

Quantum Simulation of Aqueous Ionic Clusters

Classical and quantum (Path Integral Monte Carlo) simulations are performed at several temperatures (i.e., 100, 200, and 300 K) on the ionic clusters Cl{sup -}(H{sub 2}O){sub n}, n = 1-6, to determine the importance of quantum effects on nuclear motion. The polarizable water and ion-water potential models are used to describe water-water and ion-water interactions. Comparison of classical and quantum binding enthalpies indicate that the quantum effect is significant at lower temperature (i.e., at 100 K); the difference can amount up to 20% of the total energy. The calculated quantum radial distribution functions, due to delocalization of quantum nuclei, are less structured and slightly shifted outward compared to the corresponding classical description. 16 refs., 5 figs.

Comparison of a fixed-charge and a polarizable water model

Molecular dynamics simulations are used to examine two models for water. The first model is the fixed-charge model introduced by Stillinger and Rahman and the second model is the polarizable model developed by Dang. The site–site, intermolecular pair distribution functions and the hydrogen bond lifetimes are determined for three fluid states for which neutron-diffraction determined pair functions exist. The trends in the pair functions and the bond lifetimes with decreasing density and increasing temperature for both models are similar, with the fixed-charge model pair functions changing more slowly, and the bond lifetimes more rapidly than those for the polarizable model. For the temperatures and densities examined, the explicit inclusion of polarization in the interactions appears to have a relatively small effect on the pair functions.

Accurate Solvation Free Energies of Acetate and Methylammonium Ions Calculated with a Polarizable Water Model

Accurate Solvation Free Energies of Acetate and Methylammonium Ions Calculated with a Polarizable Water Model

Potential of mean force for the methane–methane pair in water

Molecular dynamics and potential of mean force techniques are used to study methane–methane association in water. The five-site, methane–methane pair potential is taken from recent work by Spellmeyer and Kollman, while the extended simple point charge model is used to model the water–water interactions. The calculated potentials of mean force at 300 and 330 K indicate that the contact wells are significantly deeper than the corresponding solvent-separated wells. The effect of the temperature on the calculated potential of mean force is found to be small. The calculated equilibrium constants suggest that the probability of contact and the solvent-separated pairs are comparable. The shape of our potential of mean force result at room temperature using the extended simple point charge model is very similar to the results of simulations reported by Belle and Wodak using a polarizable water model. It appears that further research is necessary to assess the role of nonadditive effects in modeling of hydrophobic interactions.

An Anisotropic Polarizable Water Model: Incorporation of All-Atom Polarizabilities into Molecular Mechanics Force Fields

An anisotropic polarizable water model has been constructed using atomic polarizabilities of oxygen and hydrogen and the SPC molecular mechanics force field. The presence of polarizable interaction sites on all atoms allows treatment of dipole-dipole interactions between bonded atoms and simplifies the incorporation of polarization treatments into existing molecular mechanics force fields. The resulting many-body potential yields the correct dipole moments for the water monomer and dimer as well as molecules in the bulk. Furthermore, the model predicts structures, energies, and dynamical behavior for bulk water which are in good agreement with experimental findings. Several potential energy functions for water are currently used in computer simulations of pure water and water-solute systems. Most simulations use potentials that are pairwise additive.' Calculations with many-body potentia1s2-l6 are not common partly due to the substantial increase in computational requirements demanded by such potentials. These potentials often express the many-body component of the potential energy function in terms of atomic and molecular polarizabilities. The development of such functions and their use in simulations requires development of new molecular mechanics force fieldsz4 or reparametrization of terms in the existing force Other approaches to polarization use either additional variable point chargesg-12 or multipole expansions of the electrostatic intera~tion.~~J~ Additional many-body potentials will undoubtedly be devised as ever increasing computational ~apabilitiesl~-~ ~ encourage their use. There is a need for an all-atom, many-body potential energy function which combines polarization effects within a framework compatible with existing molecular mechanics force fields. First, many-body polarization effects are presumed to be important for describing the energetics of interfacial phenomena,20J some types of ionic2q22-26 and hydrophobic solvati~n,~~-~~ and cluster chemistry32-35 to name a few applications, although it is not yet well established for which systems polarization must be included e~plicitly.29~~~ Second, by retaining other (nonelectrostatic) components of standard molecular force fields in the polarizable potential, it may be possible to build upon the large effort that has gone into the construction of existing molecular mechanics potential functions. With the polarizable potential it becomes possible to examine the many-body nature of the electrostatic interactions. Preservation of the nonelectrostatic pairwise terms simplifies the analysis involved when comparing predictions obtained with the polarizable potential to those calculated using pairwise potentials.

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 simulations using the extended simple point charge (SPC/E), nonpolarizable water model [J. Phys. Chem. 91, 6296 (1987)]. The dependence on polarizability is found to be small, yet measurable, with the largest effects seen in the solvation structure around the highly polarizable chlorine anion. This work validates the use of some nonpolarizable water models in simulations of many condensed-phase systems of chemical and biochemical interest.

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 solvation states for the ion, and are found to be dependent on the magnitude of the Cl− polarizability assumed in the simulations.

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, our simulations place the iodine anion on the surface of the water clusters. This study demonstrates the usefulness of the molecular dynamics technique and provides a detailed picture of the ion solvation in clusters.

Calculations of hydration entropies of alkali and halide ions based on the continuum approach

A mean field approach based on continuum electrostatics is applied to the evaluation of entropies of hydration of alkali and halide ions. A statistical mechanical model we employed includes an ion and a single water molecule interacting with each other through Coulomb and Lennard-Jones potentials and represents the rest of the solvent as a dielectric continuum. TΔS of hydration values calculated with a polarizable water model agree with experimental results within 1.5 kcal/mol. Calculations with a nonpolarizablc water model lead to maximum errors of over 3 kcal/mol

Molecular dynamics study of methane hydration and methane association in a polarizable water phase

Molecular dynamics simulations are used to investigate the influence of adding an explicit polarization term to the water potential on the structural and dynamic properties of aqueous methane solutions and on methane-methane association in water. Calculations are performed using respectively two different water models: the three-center polarizable water model (PSPC), where the many-body effects are treated explicitly using the extended Lagrangian method, and the mean-field classical SPC model. Comparing the results obtained from the two sets of calculations the authors find that electronic polarization of the water molecules has a subtle though significant influence on the structure and dynamic of water molecules surrounding the hydrophobic solute. But its most remarkable influence by far is on the methane-methane potential of mean force: addition of the polarization term to the water potential effectively abolishes the much questioned solvent-separated minimum found in many previous studies with nonpolarizable water models. Polarizable water models thus appear to yield an improved physical picture of the system and should be well suited for investigating the process of hydrophobic association. 45 refs., 5 figs., 1 tab.