Rigid Water Models Research Articles

An EXAFS study of solvation and ion pairing in aqueous strontium solutions to 300°C

X-ray absorption fine structure (EXAFS) measurements on 0.10-m strontium-containing solutions from 25 to 300°C at saturated vapour pressure indicate that the first shell Sr 2+–oxygen (water) bond length decreases from 2.57 to 2.52 (±0.01) Å. Over the same temperature range, the number of coordinated first shell water molecules decreases from 8 to 6. Simulations of a 1.35-m SrCl 2 using both rigid (SPC) and flexible (BJH) water models over the range from 25 to 330°C also show a decrease in the Sr 2+–oxygen distance of 0.03 Å, which is independent of density. EXAFS measurements and molecular dynamics simulations of concentrated SrCl 2 solutions up to 270°C and 300°C, respectively, show the presence of polynuclear cluster molecules, further underlining the importance of such species in defining the solute chemistry of hydrothermal solutions in the earth’s crust. These data represent the first direct spectroscopic confirmation of solute clustering in high temperature aqueous solutions of alkali metal or alkali earth metal salts.

The effect of an external electric field on the structure of liquid water using molecular dynamics simulations

Using molecular dynamics simulations with the rigid TIP4P water model, we have analyzed the structural change of liquid water induced by an external electric field. The temperature was controlled with a Nosé–Hoover thermostat. In this paper, we report the acquisition of liquid water with the enhanced structural regularity by applying an electric field. From the simulations under various strengths of the electric field, we can see that the threshold for the significant structural change is thought to be between 0.2 and 0.15 V/Å. When the number of six-membered rings is increased by the external electric field, so that water is forced to have structural regularity, we calculate the diffusion coefficients and discover that water we make in the simulations is not solid but still liquid under the electric field.

Comparison of rigid and flexible simple point charge water models at supercritical conditions

This study investigates the differences between the predictions of various properties of rigid and flexible simple point charge water models at supercritical conditions. Molecular dynamics simulations were conducted for supercritical water in a temperature range of 773-1073 K and densities in the range 115-659 kg/m3. We present thermodynamic data, pair correlation functions, selfdiffusivity, power spectra, dielectric constants, and variaous measures of hydrogen bonding at different state conditions. The flexible water model performs better in predicting the pressures along the supercritical isotherms simulated. Agreement between experimental and calculated dielectric constants is superior for the flexible water model, particularly at high densities. The flexible model exhibits a greater degree of hydrogen bonding and more persistent hydrogen bonds than does the rigid model. The structural features of supercritical water at high densities are identical for the two water models. At low densities, however, the flexible potential exhibits pair correlation functions with enhanced peaks. Inclusion of flexibility in the potential model does not result in a significant shift in the position of the rotational/librational peak in the power spectrum. The self-diffusivities obtained from the simulations are within the accuracy of the experimental values for both the rigid and flexible models. On balance the inclusion of flexibility improves agreement with the properties of real supercritical water while incurring little or no additional

Comparison of rigid and flexible simple point charge water models at supercritical conditions

Comparison of rigid and flexible simple point charge water models at supercritical conditions

Molecular dynamics simulation studies of the mercury-water interface

Molecular dynamics (MD) simulation studies of the mercury-water interface are presented. A slab of water molecules, about 25Åwide, confined by mercury phases, is investigated. A rigid water model and a flexible water model are used to describe the water-water interactions. Recently, we parameterized ab initio calculations on mercury-water clusters to describe the mercury-water potential. The liquid mercury phase is approximated as a rhombohedral lattice. The mercury-mercury interaction is modelled by a harmonic potential. Our results show a significant influence of the mercury phase on the structural and dynamic properties of the water phase. The structure of the interface is analysed in terms of density functions, radial distribution functions, orientation of the water molecules, potential drop, and hydrogen bonding characteristics.

The Second Virial Coefficients for Water from Monte Carlo Integration Using Reduced Effective Charges

Abstract The reduction of the dipole moment of effective pair potentials of rigid water models enables accurate second virial coefficients to be determined over the whole temperature range. Subsequent Isothermal Isobaric (NPT) ensemble simulations of water vapour have shown that there is a significant difference in vapour densities between the modified water potentials.

Settle: An analytical version of the SHAKE and RATTLE algorithm for rigid water models

AbstractAn analytical algorithm, called SETTLE, for resetting the positions and velocities to satisfy the holonomic constraints on the rigid water model is presented. This method is still based on the Cartesian coordinate system and can be used in place of SHAKE and RATTLE. We implemented this algorithm in the SPASMS package of molecular mechanics and dynamics. Several series of molecular dynamics simulations were carried out to examine the performance of the new algorithm in comparison with the original RATTLE method. It was found that SETTLE is of higher accuracy and is faster than RATTLE with reasonable tolerances by three to nine times on a scalar machine. Furthermore, the performance improvement ranged from factors of 26 to 98 on a vector machine since the method presented is not iterative. © 1992 by John Wiley & Sons, Inc.

Potential of mean force by constrained molecular dynamics: A sodium chloride ion-pair in water

The constrained molecular dynamics simulation method has been used to obtain the mean force and the mean force potential between two particles in solution. The method has been tested for a Lennard-Jones liquid and applied to the study of a Na +-Cl − ion-pair in aqueous solution. A flexible SPC model for water has been assumed. The results have been interpreted in the light of the solvent structure around the ions for separations corresponding to the maxima and minima of the mean force potential. In contrast to earlier studies using rigid water models, the solvent separated ion pair configurations are more stable than the contact ion pair configurations.

A molecular dynamics study of polarizable water

We have investigated the effect of adding a point polarizability to a SPC like rigid water model in molecular dynamics simulations. A new algorithm for calculating the induced dipole moments based on a predictive, instead of an iterative scheme, is presented. The predictive scheme considerably reduces the demand on computer time. Both schemes gives identical results for energy, structure and dynamics. In the liquid phase simulations of the polarizable water model the total dipole moment is enhanced from its static gas phase value of 1·85 D to 3·2 D. The radial distribution functions indicate an increased structure as compared to SPC water. Dynamic properties are slower than for the original SPC model. Further adjustments of the hard core of the polarizable SPC model to yield a better energy estimate reduces the total dipole moment to 2·9 D. This model, the polarizable SPC (PSPC) water, enhances the agreement with the original SPC structure and improves the dynamical properties of the model. In order to si...

Hydrogen bond cooperativity in simulated water: Time dependence analysis of pair interactions

Hydrogen bonding in water systems is investigated by introducing a new method to analyze the time dependence of pair-interaction data obtained from a molecular dynamics simulation of 216 ST2 [F. H. Stillinger and A. Rahman, J. Chem. Phys. 60, 1545 (1974)] water molecules at 280 K. This approach avoids the use of cutoff values and yields a more realistic bond population, whose distributions of geometric and energetic properties are reported as a function of the bond lifetimes. For the fraction of long-lived bonds, correlation among bond stability, molecular mobility, and local structure is elecited. Percolation analysis of HB network evidences cooperativity in the spatial distribution of bonds, which does not originate from proton polarizability of HB and/or from many-body terms of the interaction potentials since a rigid water model and pair potentials are used. These features can play a role in the anomalous properties of liquid water.

Solute-induced Water Structure: Computer Simulation on a Model System

Abstract Two series of Monte Carlo simulations have been carried out on a system consisting of 125 water molecules, one of which is kept fixed to simulate a water molecule whose mobility is restricted by a solute. The results are checked against similar simulations without restrictions, used as a control, and they show how the blocked molecule helps increase both the structural order and the connectivity of the hydrogen bond network. Cooperativity originating from proton polarizability of H-bonds and/or from many-body terms of interaction potentials cannot be involved since we use a rigid water model and ab initio pair potentials. The present findings are interpreted as indicative of a motion-dependent effect which is triggered by the blocked molecule and propagates along H-bonds. As has already been proposed, this effect might play a role in water-mediated solute—solute interactions.