Water Interaction Potential Research Articles

Shape and size of simple cations in aqueous solutions: A theoretical reexamination of the hydrated ion via computer simulations

The simplest representation of monoatomic cations in aqueous solutions by means of a sphere with a radius chosen on the basis of a well-defined property (that of the bare ion or its hydrate) is reexamined considering classical molecular dynamics simulations. Two charged sphere–water interaction potentials were employed to mimic the bare and hydrated cation in a sample of 512 water molecules. Short-range interactions of trivalent cations were described by Lennard-Jones potentials which were fitted from ab initio calculations. Five statistically independent runs of 150 ps for each of the trivalent spheres in water were carried out in the microcanonical ensemble. A comparison of structural and dynamical properties of these simple ion models in solution with those of a system containing the Cr3+ hydrate ([Cr(H2O)6]3+) is made to get insight into the size and shape definition of simple ions in water, especially those that are highly charged. Advantages and shortcomings of using simple spherical approaches are discussed on the basis of reference calculations performed with a more rigorous hydrated ion model [J. Phys. Chem. B 102, 3272 (1998)]. The importance of nonspherical shape for the hydrate of highly charged ions is stressed and it is paradoxically shown that when spherical shape is retained, the big sphere representing the hydrate leads to results of ionic solution worse than those obtained with the small sphere. A low-cost method to generate hydrated ion–water interaction potentials taking into account the shape of the ionic aggregate is proposed.

New Effective Potentials Extraction Method for the Interaction between Cations and Water

A very simple method for the extraction of effective interaction potentials from ab initio calculations was proposed (Periole et al. J. Phys. Chem. 1997, 101, 5018), and simple two-body cation−water interaction potentials were derived for several cations, Li+, Na+, K+, Be2+, Mg2+, and Ca2+, using two facts: first, water molecules in the close vicinity of cations are strongly structured and present a constrained orientation towards the ion; second, at larger distances the ion-water interaction is mainly electrostatic. In the present work, an extension to Rb+ and Sr2+ and some refinements of this method are presented. In particular, we explore the most adequate way of including the nonadditivity and polarization effects that arise from the ion-water-water and water-water interactions. The potentials obtained with the new extraction methods are compared with the empirical potentials of Åqvist (Åqvist, J. J. Phys. Chem. 1990, 94, 8021) that were adjusted to reproduce experimental data. Those obtained with the exploration−TIE method are also tested by performing molecular dynamics simulations of the various cation−water systems and the results are found to be in good agreement with experimental data. In particular, they yield cation hydration free energy differences (ΔG values) that are, in general, in good accordance with experimental figures. This latter method is ideally suited and easy to apply to obtain effective interaction potentials for molecular systems with restricted geometric conditions that appear in numerical simulations, either Monte Carlo or molecular dynamics.

A molecular dynamics study of the Cr3+ hydration based on a fully flexible hydrated ion model

A theoretical study of the Cr3+ hydration in aqueous solutions has been carried out by means of molecular dynamics (MD) simulations. Ion–water intermolecular interaction potentials are based on first principles using the idea of the previously developed hydrated ion–water interaction potential: The bare ion, Mn+, is replaced by its corresponding hydrate, [M(H2O)6]n+, and the water molecules interact with the hydrate by means of an ab initio [M(H2O)6]n+–H2O interaction potential. A new ab initio interaction potential has been developed to describe the Mn+–(H2O)first-shell interaction based on an examination of the hexahydrate potential-energy surface section that distorts the position of one of the cluster water molecules, the remaining five fixed at their equilibrium position. These two complementary interaction potentials, which describe ion–water interactions have been combined with the TIP4P model for water molecules. Structural and dynamical results derived from the analysis of 1 ns of simulation for a sample formed by [Cr(H2O)6]3+ and 512 H2O are presented. Rigidity effects of the cluster are examined by comparing the present results with those previously obtained with a model of rigid hexahydrate [J. Phys. Chem. B 102, 3272 (1998)]. A new definition of hydrated ion based on the rotational properties of its hydrate is supported.

Dynamics of a Highly Charged Ion in Aqueous Solutions: MD Simulations of Dilute CrCl3 Aqueous Solutions Using Interaction Potentials Based on the Hydrated Ion Concept

Structural and dynamical properties of dilute aqueous solutions containing a trivalent cation have been determined by means of Molecular Dynamics simulations. The concept of hydrated ion has been used when considering aqueous solutions of Cr3+, [Cr(H2O)6]3+ being the cationic entity interacting in solution. An ab initio Cr3+ hydrate−water interaction potential previously developed [J. Phys. Chem. 1996, 100, 11748] and a new one describing the Cr3+ hydrate−Cl- interactions have been used with a TIP4P water model to carry out simulations of the system Cr(H2O)6Cl3 + 512H2O. To examine the role of anions, simulations without chloride ions were performed as well ([Cr(H2O)6]3+ + 512H2O). To investigate the influence of shape and size of the hydrated cation, two additional models of trivalent cation have been studied using the simplest concept of spherical ion. Ad hoc charged sphere−water interaction potentials for the latter situations were built. RDFs, hydration numbers, vibrational spectra of the intermolecul...

New basis set superposition error free ab initio MO-VB interaction potential: Molecular-dynamics simulation of water at critical and supercritical conditions

Recently, a controversy has come to light in literature regarding the structure of water in nonambient conditions. Disagreement is evident between the site–site pair correlation functions of water derived from neutron diffraction and those obtained by computer simulations which employ effective pairwise potentials to express the intermolecular interactions. In this paper the SCFMI method (self-consistent field for molecular interaction) followed by nonorthogonal CI (configuration interaction) calculations was used to determine a new water–water interaction potential, which is BSSE (basis set superposition error) free in an a priori fashion. Extensive calculations were performed on water dimer and trimer and a new parametrization of a NCC-like (Niesar–Corongiu–Clementi) potential was accomplished. This was employed in the molecular-dynamics simulation of water. The effect of temperature and density variations was examined. Acceptable agreement between site–site correlation functions derived from neutron diffraction data and from computer simulation was reached. In particular, a weakening of the hydrogen bonded structure was observed on approaching the critical point, which reproduces the experimental behavior. The simulations were performed using the MOTECC (modern techniques in computational chemistry) suite of programs. The present results show the importance of BSSE-free nonorthogonal orbitals in an accurate description of the intermolecular potential of water.

Interaction potential of Al3+ in water from first principles calculations

We present a parametrization of the interaction potential for Al3+ in water from first principles calculations. We have performed a critical study of the Al3+–water interaction using sequences of correlation consistent basis sets that approach the complete basis set limit and include core-valence correlation effects. We suggest as minimum theoretical requirements treatment of the electron correlation at the MP2 level of theory using a triple zeta quality basis set that accounts for the effect of core-valence correlation. The latter amounts for an increase of ∼5 kcal/mol (3%) to the stabilization energy, a shortening of 0.015 Å in the Al–O distance, and an increase of 22 cm−1 in the harmonic frequency of the Al–O vibration. This is the first time that core-valence effects were investigated for this system. The stabilization energy of the Al3+(H2O) cluster is 201 kcal/mol and the corresponding Al–O bond length is 1.719 Å at the MP2 level of theory with the cc-pwCVQZ basis set. This minimum is metastable with respect to the Al2++H2O+ asymptote since even the second ionization potential (IP) of Al is larger than the first IP of water. The hexa-aqua cluster Al3+(H2O)6 is, however, stable upon dissociation to Al3+(H2O)5+H2O by 64.8 kcal/mol, demonstrating the capacity of “effective” solvation in stabilizing the charge on the cation. The optimal structures of the n=5 and 6 clusters (having C2v and Th symmetries, respectively) and their harmonic vibrational frequencies are the first ones reported at the MP2 level with basis sets of this size. Core-valence correlation effects for the n=6 cluster are found to be of similar magnitude with those observed for the n=1 cluster. The stabilization energy of the n=6 cluster with respect to its fragments is 723.7 kcal/mol and the corresponding Al–O distance is 1.911 Å. These results were used in order to parametrize a pairwise-additive interaction potential for aluminum–water interaction that was grafted onto the Toukan–Rahman interaction potential for water. The potential model reproduces the ab initio results for Al3+(H2O)6 within 2.0 kcal/mol for the stabilization energy and 0.003 Å for R(Al–O) distance. Using this potential we estimated the enthalpy of solvation of Al3+ to be −1106±6 kcal/mol, therefore favoring the lower value of the experimentally obtained data (−1115 and −1140 kcal/mol, respectively). In addition, we calculate the first peak of the Al–O radial distribution function at 1.885 Å, in excellent agreement with x-ray diffraction studies that suggest a peak at 1.882±0.004 Å. We compute the first peak of the Al–H radial distribution function at 2.473 Å and the average angle between the plane of a water molecule and the Al–O vector at −28.27°.

Simple Two-Body Cation−Water Interaction Potentials Derived from ab Initio Calculations. Comparison to Results Obtained with an Empirical Approach

Ab initio calculations were performed on M(H2O)n systems, M being Li+, Na+, K+, Be2+, Mg2+, or Ca2+, with n = 1, 2, 4, or 6. For the most hydrated systems, parameters for the effective Lennard-Jones interaction between the cation and the water molecules were determined, so as to reproduce ab initio results. In order to compare our results to those obtained previously by J. Åqvist with a purely empirical approach, water−water interactions were assumed to be given by the TIP3P model. Different forms for the effective two-body interaction potential were tested. The best fits of ab initio data were obtained with a smooth r-7 repulsive and a classical r-4 attractive term, in addition to standard Coulombic interactions. Though better fits were obtained for alkaline cations than for alkaline-earth ones, only Be2+ obviously requires a more complicated form of the potential energy function.The corresponding parameters were tested with molecular dynamics simulations of cations in water solutions and with hydration free energy difference calculations, using the thermodynamic perturbation approach. Radial distribution functions consistent with experimental data were obtained for all cations. Free energy differences are obviously much more challenging. The most accurately reproduced value is the difference between the hydration free energies of Na+ and K+. This result is likely to be significant since effective interaction energies between Na+ or K+ and water molecules as obtained in Åqvist's and in the present work are found to be very similar, despite the fact that the corresponding sets of parameters were determined with completely different approaches.

Bridge over troubled water: the apparent discrepancy between simulated and experimental non-ambient water structure

Recent neutron diffraction experiments, which exploit hydrogen isotope substitution techniques to extract the HH, OH and OO site - site radial distribution functions for water, indicate that as the temperature of water is raised above the critical point, the hydrogen-bonding network, as measured by the height of the first peak in the OH distribution function, collapses. Several computer simulations, however, dispute the accuracy of the experimentally determined distribution functions: they show that the measured data cannot be obtained from any physical arrangement of water molecules. By applying additional constraints on the small-radius behaviour of the radial distribution functions, and by repeating some of the experiments under different experimental conditions, the accuracy of the extracted radial distribution functions has now been improved. The new distribution functions, while not qualitatively different from what has already been published, satisfy the fundamental objection to the previous results, namely they can be simulated with physical assemblies of water molecules. These simulated distributions of molecules indicate a weak degree of hydrogen bonding in water at 673 K which is greatly reduced compared to that in the hydrogen-bond network of ambient water. The analysis is supported by a new computer simulation of water structure using an `empirical' water interaction potential, which contains none of the long-range features of traditional charge models, such as SPC/E. This short-range potential is able to reproduce most features of the experimental data to good accuracy, even under supercritical conditions.

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.

Spinodal of liquid water.

An open question in the study of water concerns the shape of the liquid spinodal line in the phase diagram of water, a boundary which represents the limit of mechanical stability of the liquid state. It has been conjectured that the pressure of the liquid spinodal ${\mathit{P}}_{\mathit{s}}$(T) does not decrease monotonically with decreasing temperature T, but passes through a minimum and is ``reentrant'' from negative to positive pressure P in a region of T in which the liquid is deeply supercooled. The conjectured minimum in ${\mathit{P}}_{\mathit{s}}$(T) has not been directly observed due to the difficulties encountered in experiments which attempt to study liquid water under tension. Here we exploit the ability of molecular-dynamics computer simulations to model the behavior of liquid water deep into its metastable region. We thereby attempt to observe a minimum in ${\mathit{P}}_{\mathit{s}}$(T). We first argue that the ST2 potential of Stillinger and Rahman [J. Chem. Phys. 60, 1545 (1974)] is the best of several commonly used water interaction potentials for this purpose. Then, we conduct simulations of a system of ST2 particles over a wide range of stable and metastable liquid-state points, and demonstrate that ${\mathit{P}}_{\mathit{s}}$(T) for ST2 is not reentrant. In a second set of simulations we test if the behavior we find is limited to the ST2 potential by exploring the relevant thermodynamic region of the liquid as simulated by the TIP4P interaction potential of Jorgensen et al. [J. Chem. Phys. 79, 926 (1983)]. We find that the TIP4P potential confirms the absence of a reentrant spinodal. We also show how the structural and energetic properties of both the ST2 and TIP4P liquids are consistent with the absence of a reentrant spinodal.

Ab initio copper–water interaction potential for the simulation of aqueous solutions

AbstractA new ab initio effective two‐body potential that aims at mimicking the average copper–water interaction energy of the first solvation shell was developed. This new potential, together with the MCY water–water potential and a three‐body ion–water–water induction potential, is tested in simulations of gas‐phase clusters [Cu2+(H2O)20] and diluted solutions [Cu2+(H2O)200] at T = 298 K. The results of simulations with conventional ab initio pair potentials, with and without three‐body induction corrections, are also presented. The different types of copper–water interaction potentials are evaluated comparatively and the efficiency of the newly proposed effective pair potential is discussed. © 1993 John Wiley & Sons, Inc.

Monte Carlo study of the aqueous hydration of dimethylphosphate conformations

AbstractMonte Carlo computer simulations were performed on dilute aqueous solutions of the dimethylphosphate anion and the sodium dimethylphosphate ion pair, with the two phosphodiester torsional angles in the gauche–gauche, gauche–trans, and trans–trans conformations. The structural and energetic aspects of the aqueous hydration of each molecule were analyzed in terms of quasi component distribution functions based on the proximity criterion and partitioned into ionic, hydrophilic, and hydrophobic contributions to facilitate an understanding of the hydration pattern and conformational trends in these multifunctional solutes. Special attention was also paid to methodological issues affecting hydration, such as statistical uncertainty in the determined hydration indices, choice of partial atomic charges for the solute atoms, and solute–water interaction potentials adopted in the simulations. The results showed that gauche–trans and gauche–gauche forms are equally favorable for the dimethylphosphate anion with the trans extended form destabilized by hydration. The sodium dimethylphosphate ion pair hydration energetically favors the trans–trans conformation.

Revisiting small clusters of water molecules

The geometries and relative energies of small clusters of water molecules, (H 2O) n with 4 ⩽ n ⩽ 8, are reported. For each value of n we have considered the conformations corresponding to the lowest-energy minimum and those in nearby relative minima. Thus we report on six tetramers, four pentamers, six hexanlers, four heptamers, and eigth octamers. The geometrical conformations have been obtained using the Metropolis Monte Carlo method as a minimization technique, where the interaction energy is computed with the MCY potential plus three- and four-body corrections previously discussed. All the reported structures for a given cluster size are found to be close in energy. For the lowest conformation the geometry was optimized with ab initio SCF computations using energy gradients. Our results are compared with previous theoretical studies. We discuss the convergence of the interaction potential for liquid water when expressed in terms of a many-body series expansion.

Water–water interaction potential: An approximation of the electron correlation contribution by a functional of the SCF density matrix

The approximate expression for the correlation energy suggested by Colle and Salvetti (1975), but modified with a different set of parameters obtained by a more accurate description of the Coulomb hole, has been applied to approximate the electron correlation in the water two-body interaction potential. The results obtained are in good agreement with previous configuration interaction and perturbative computations, indicating that the approach can be applied successfully to calculate the correlation contribution in interaction potentials between large molecules with a reasonable computational effort. With the new two-body potential, a Monte Carlo simulation of liquid water has been carried out and a comparison of computed and experimental results is presented for the internal energy, the oxygen–oxygen radial distribution function, and the neutron scattering structure function.

Molecular dynamics simulation of water near walls using an improved wall—water interaction potential

Two molecular dynamics simulations, lasting 14 and 24 ps, have been performed with 150 water molecules between two walls. The wall potential is obtaine

Ion–water interaction potentials for alkali metal cations and halide anions

Empirical potential parameters for ion–water interactions have been constructed by three simple methods and successfully used for describing ion hydration. In the first two methods, we have computed the range parameter (ρij) by fixing the energy parameter at a particular value, in a potential of the form EREP =Aij exp(−Rij/ρij). For this purpose, we used either the experimental ion–water distances or one of the experimental enthalpies (ΔH0, 1). In the third method, we have developed a simple correlation scheme by which both the parameters (Aij and ρij) can be computed. Although the parameters obtained by all the three methods seem to describe the gas phase hydration energies equally well, the last method seems to be the best as it allows the computation of both the parameters. These procedures are useful in reproducing the experimental energetics from the experimental ion–water distances and vice versa without requiring parameters taken from other sources. The consistency of one can be checked by the other, independently. The range parameter for the repulsive interaction has been shown to be the most sensitive of all the parameters and hence plays a significant role in deciding the magnitudes of the energies and geometries for the hydration of the ions. Parameters derived from scattering experiments have also been used to describe the gas phase hydration energies but they are found to be unsuccessful in producing consistent results. The major result of the present investigations is that a simple correlation scheme can be developed for describing all interactions. This type of analogy is extremely helpful especially for explaining ion–ion, ion–atom, atom–atom, atom–molecule, molecule–molecule, or ion–molecule interactions.

Size effects in aerosol particle interactions: The Van der Waals potential and collision rates

Three effects which are explicitly dependent on aerosol particle size are identified and discussed. They are focussed about the particle collision rate and how it relates to the properties of the gas, the particle, and the particle's interaction potential energy which play roles in particle-particle collision rates. By incorporating the conduction electronic free path effect for conductors into the frequency-dependent dielectric constants of silver and graphite, particle size effects in the Lifshitz-Van der Waals potentials for identical pairs of 1 nm and 100 nm particles are evaluated. Water and tetradecane particle interaction potentials for the same size particles are also calculated to illustrate size effects due to the retardation of the interaction. These potentials are then used to calculate the enhancement of the particle collision rates above their value in the absence of any potential at various gas pressures. The roles of the interaction potential in collision among identical pairs of particles of differing compositions is also briefly discussed.

Study of the structure of molecular complexes. XIII. Monte Carlo simulation of liquid water with a configuration interaction pair potential

A water–water interaction potential obtained from configuration interaction calculations has been used to simulate liquid water, at 25 °C, by a Monte Carlo technique. The resulting radial distribution functions and x-ray and neutron scattering intensities are compared with experiment and found to be in satisfactory agreement. Some thermodynamic properties are also computed and discussed. The overall agreement seems to indicate that many-body effects contribute little in determining the structure of liquid water, although they seem to be important for an accurate simulation of internal energy and related quantities.