Water Pair Potential Research Articles

On the hydration structure of LiCl aqueous solutions: A Reverse Monte Carlo based combination of diffraction data and Molecular Dynamics simulations

Reverse Monte Carlo (RMC) calculations have been carried out on aqueous LiCl solutions at several concentrations, comparing experimental X-ray and neutron diffraction data with partial radial distribution functions (prdf) from Molecular Dynamics (MD) simulations based on conventional ion–ion, ion–water, and water–water pair potentials. The RMC calculations indicate that the hydration shell of the Li + ions could be even more structured than computer simulations predict, containing at high concentrations unusually few water molecules, of the order of 2 to 3. Concerning the consistency between the structures based on potential models and diffraction data: the ion–water partial radial distribution functions (prdf) from MD are found to be consistent with the experimental data whereas the water–water and ion–ion partials are more problematic. The most serious deficiency is that at high salt concentrations the O–O prdf cannot be made consistent with the present X-ray diffraction data.

An accurate analytic representation of the water pair potential

The ab initio water dimer interaction energies obtained from coupled cluster calculations and used in the CC-pol water pair potential (Bukowski et al., Science, 2007, 315, 1249) have been refitted to a site-site form containing eight symmetry-independent sites in each monomer and denoted as CC-pol-8s. Initially, the site-site functions were assumed in a B-spline form, which allowed a precise optimization of the positions of the sites. Next, these functions were assumed in the standard exponential plus inverse powers form. The root mean square error of the CC-pol-8s fit with respect to the 2510 ab initio points is 0.10 kcal mol(-1), compared to 0.42 kcal mol(-1) of the CC-pol fit (0.010 kcal mol(-1) compared to 0.089 kcal mol(-1) for points with negative interaction energies). The energies of the stationary points in the CC-pol-8s potential are considerably more accurate than in the case of CC-pol. The water dimer vibration-rotation-tunneling spectrum predicted by the CC-pol-8s potential agrees substantially and systematically better with experiment than the already very accurate spectrum predicted by CC-pol, while specific features that could not be accurately predicted previously now agree very well with experiment. This shows that the uncertainties of the fit were the largest source of error in the previous predictions and that the present potential sets a new standard of accuracy in investigations of the water dimer.

New model pair potential for water. Effect of inclusion of specific O⋯H interactions on the topology and dynamics of the hydrogen-bond network

Classical molecular dynamics was used to calculate the topologic and dynamic characteristics of H-bond networks in water, using various model potentials. The explicit inclusion of intermolecular interactions of oxygen and hydrogen atoms leads to a model involving additional stabilization of tetrahedrally coordinated molecules and, as a consequence, increased number of such molecules in the system. The H-bond lifetimes point to a strongly correlated molecular motion in the immediate environment.

A study of aqueous solutions of lanthanide ions by molecular dynamics simulation with ab initio effective pair potentials

Infinitely dilute aqueous solutions of Nd3+, Gd3+, and Yb3+ have been studied with molecular dynamics simulations, using ab initio effective ion–water pair potentials based on the polarizable continuum model. Structural results, as first peak positions of g(r) are in good agreement with experimental data. We obtain a coordination number of 9 for all the cations. This value agrees with experimental measurements for Nd3+ and Gd3+ but overestimates them for Yb3+. Significant differences between Yb3+ and the other two ions have however been observed in a detailed analysis of the solvation shell structure, based on the diagonalization of the inertia tensor. A similarity index based on a linear combination of the inertia tensor eigenvalues is proposed. Going from Nd3+ to Yb3+, structures like a capped square antiprism (CSQA) are more favored than a tricapped trigonal prism. In contrast to the lanthanide contraction observed for the most probable ion–oxygen distances, in CSQA structures the distance from the center of mass of the polyhedron to the capped O increases from Nd3+ to Yb3+. Dynamics of internal rearrangements has been studied and residence times of waters in the first and second hydration shell have been calculated. Diffusion coefficients for the ions appear somewhat underestimated with respect to the experimental data, which agree with the value we obtain for hydration shell waters. Linear and angular velocity correlation functions and spectra of first shell waters show a large increase of intensity at high frequency compared to bulk solvent. The Yb3+ solution consistently presents these features in enhanced form.

How to build a better pair potential for water

With the objective of improving the effective pair potentials for water, we develop a potential model that employs diffuse charges, in addition to the usual point charges, on the oxygen and hydrogen atoms, to account for charge penetration effects. The potential has better transferability from the liquid to gaseous phases since, unlike many existing models, it does not require an enhanced dipole moment. As a result it accurately reproduces the structural and thermodynamic properties of water over a wide range of conditions. Moreover, by allowing for electronic polarization when evaluating the total dipole moment of the simulated fluid, the model leads to the correct value of the dielectric constant for virtually any state point. At room temperature the calculation produces an average dipole moment of 3.09 D, in accord with recent theoretical and experimental evaluations. This supports the idea that induction effects in water are more important than previously expected.

Water pair potential of near spectroscopic accuracy. II. Vibration–rotation–tunneling levels of the water dimer

Nearly exact six-dimensional quantum calculations of the vibration–rotation–tunneling (VRT) levels of the water dimer for values of the rotational quantum numbers J and K ⩽2 show that the SAPT-5s water pair potential presented in the preceding paper (paper I) gives a good representation of the experimental high-resolution far-infrared spectrum of the water dimer. After analyzing the sensitivity of the transition frequencies with respect to the linear parameters in the potential we could further improve this potential by using only one of the experimentally determined tunneling splittings of the ground state in (H2O)2. The accuracy of the resulting water pair potential, SAPT-5st, is established by comparison with the spectroscopic data of both (H2O)2 and (D2O)2: ground and excited state tunneling splittings and rotational constants, as well as the frequencies of the intermolecular vibrations.

Water pair potential of near spectroscopic accuracy. I. Analysis of potential surface and virial coefficients

A new ab initio pair potential for water was generated by fitting 2510 interaction energies computed by the use of symmetry-adapted perturbation theory (SAPT). The new site–site functional form, named SAPT-5s, is simple enough to be applied in molecular simulations of condensed phases and at the same time reproduces the computed points with accuracy exceeding that of the elaborate SAPT-pp functional form used earlier [J. Chem. Phys. 107, 4207 (1997)]. SAPT-5s has been shown to quantitatively predict the water dimer spectra, see the following paper (paper II). It also gives the second virial coefficient in excellent agreement with experiment. Features of the water dimer potential energy surface have been analyzed using SAPT-5s. Average values of powers of the intermolecular separation—obtained from the ground-state rovibrational wave function computed in the SAPT-5s potential—have been combined with measured values to obtain a new empirical estimate of the equilibrium O–O separation equal to 5.50±0.01 bohr, significantly shorter than the previously accepted value. The residual errors in the SAPT-5s potential have been estimated by comparison to recent large-scale extrapolated ab initio calculations for water dimer. This estimate—together with the dissociation energy D0 computed from SAPT-5s—leads to a new prediction of the limit value of D0 equal to 1165±54 cm−1, close to but significantly more accurate than the best empirical value.

Cu+ in Liquid Ammonia and in Water: Intermolecular Potential Function and Monte Carlo Simulation

The solvation structure of Cu+ in water and in liquid ammonia has been investigated using the Metropolis Monte Carlo method. The systems consisting of one Cu+ in 215 solvent molecules have been simulated at a temperature of 240 K for ammonia and 298 K for water, respectively. Cu+−ammonia and Cu+−water pair potentials have been newly developed based on ab initio calculations of double-ζ quality. Structural properties were investigated by means of radial distribution functions and their running integration numbers, leading for the first solvation shell to an average coordination number 6 and Cu−N distance of 2.20 A in ammonia, and to number 6 and Cu−O distance of 2.20 A in water. The RDFs, coordination number distributions, and pair interaction energy distribution analyses indicate that ligand exchange reactions take place more easily in water than in liquid ammonia.

Spectroscopic determination of the water pair potential.

A polarizable water pair potential was determined by fitting a potential form to microwave, terahertz, and mid-infrared (D2O)2 spectra through a rigorous calculation of the water dimer eigenstates. It accurately reproduces most ground state vibration-rotation-tunneling spectra and yields excellent second viral coefficients. The calculated dimer structure and dipole moment are very close to those determined from microwave spectroscopy and high-level ab initio calculations. The dimer binding energy and acceptor switching and donor-acceptor interchange tunneling barriers are in excellent agreement with recent ab initio theory, as are cyclic water trimer and tetramer structures and binding energies.

Structure and property prediction of sub- and supercritical water

In this report two approaches are presented to predict the structure and PVT behavior of associating fluids with emphasis on water. One approach is the development of equations of state based on the analytic chain association theory (ACAT). An associating fluid is assumed to be a mixture of monomers, dimers, trimers, etc., for which the composition distribution is obtained. The resulting equations are simple enough to be used for PVT calculations. The second approach is the development of an effective Kihara pair potential for water which incorporates the hydrogen bonding using the ACAT. This potential function has been used in an analytical expression to predict the first shell of the radial distribution function (RDF) for water. The expression for RDF which was initially applied to simple potential energy functions, such as the Lennard–Jones and Kihara functions satisfies the general functionality of the RDF with respect to intermolecular potential, temperature and density as well as all the limiting values of RDF at high temperature and dilute gas, and infinite separation. The effective potential parameters are determined to predict the first shell of the RDF data for water at various subcritical temperatures and densities. The predicted results for water at near-critical and supercritical conditions are shown to be in agreement with the data obtained by neutron diffraction experiments and with the simulation data.

Pair potential for water from symmetry-adapted perturbation theory

The interaction energies of over a thousand water dimer configurations have been calculated using the symmetry-adapted perturbation theory. Effective, interaction optimized bases were used leading to 0.2 kcal/mol accuracy near the minimum of the dimer potential. The computed points were then fitted to two types of analytic potential energy surfaces, a site-site form and an expansion in functions dependent on the vector connecting the centers of mass and on the Euler angles defining the orientation of each monomer. The second virial coefficient was calculated from these surfaces including the quantum correction and isotopic dependence, as well as the molar heat capacity at constant pressure. Comparison of these data to experiment shows that both of our surfaces are superior to any previously available.

Quantum corrections to induced light scattering and the second virial coefficient of water and hydrogen sulfide

Some model pair potentials of liquid water and hydrogen sulfide have been used for computing second virial coefficients, lowdensity values of depolarized light scattering due to the dipole-induced-dipole (DID) mechanism, mean-square torques and mean-square forces versus temperature. The second virial coefficient and depolarized scattering were corrected to account for quantum effects, which are found to be relevant in the case of water. The “MCY” water pair potential gives values of the second virial coefficient in good agreement with experiment, while computed and experimental values of the DID intensity disagree significantly. Possible reasons for this discrepancy are discussed.

Pressure enhancement of gas-phase water oligomer populations: A RRHO MCY-B/EPEN computational study

Existing theoretical data, including the recently suggested MCY-B water pair potential and EPEN data on water oligomers in conjunction with rigid-rotor and harmonic-oscillator (RRHO) partition functions, are combined to evaluate equilibrium constants of water associate, (H2O) i , formation. The equilibrium constant set obtained is employed to study water cluster populations under various temperatures and pressures. Within the model it is shown that the pressure dependence of the mole fraction of a cluster in the equilibrium steam exhibits a maximum. The maximum with increasing temperature is shifted towards higher pressure values, however, its height is increasing, too. The importance of the finding for an assessment of optimum conditions for a cluster observation as well as reliability of the RRHO MCY-B model of steam are discussed.

Monte Carlo studies of the structure of NaCl aqueous solution at three different concentrations

Monte Carlo simulations have been carried out for aqueous NaCl solution at three different concentrations. The water—water interactions are described by the Matsuoka—Clementi—Yoshimine (MCY) potential, while the ion—water pair potentials have been derived from ab initio calculations. The structure of the solutions is described by radial distribution functions. The strong effects of increasing salt concentration on water structure and the successive decay of the second hydration shell are discussed.

Computer experiment on aqueous solution. IV. Molecular dynamics calculation on the hydration of urea in an infinitely dilute aqueous solution with a new urea–water pair potential

A molecular dynamics calculation on aqueous solution of urea has been carried out using constant temperature technique. The total number of molecules was 216, one of which was urea and the temperature was set to 298.15 K and an experimental value was used for the density. For water–water interaction, the MCY (Matsuoka–Clementi–Yoshimine) potential was used, whereas a new potential function was determined for urea–water interaction from SCF LCAO calculations for more than 800 different dimeric configurations with an STO-3G basis set and subsequent multiparameter fitting of the MO results thus obtained to an appropriate functional form by a nonlinear optimization method. The molecular dynamics calculation has been carried out up to 64 000 time steps and from the final 40 000 time steps, thermodynamic quantities, structural and energetic distribution functions, and time-dependent properties were obtained. The original water structure in the vicinity of urea molecule is slightly changed energetically by incorporation of the urea molecule. However, this energy difference is insignificant for the whole system. Instead of the possibility to form strong hydrogen bonding as estimated from the potential function, it is found that urea molecule could enter into the water structure without any appreciable distortion. This fact was confirmed by the angular dependence of any distribution function around the urea molecule. The hydrophilic region does not show a large energetic stabilization between water molecules and the system is stabilized slightly by including urea–water interaction. In contrast to this, the energy for water molecules in the hydrophobic region (above and below the plane containing urea molecule) becomes lower than that of pure water, although this region is small and water molecules cannot form a strong hydrogen bond with urea. This fact reveals that the role of each functional region, which may be either hydrophobic or hydrophilic, is similar to that of alcohol in aqueous solution, although the whole hydration structure of urea molecule is somewhat different from that of alcohol. Reflecting strong interaction of urea–water, the diffusion coefficient for shell water molecules in the vicinity of urea (within 5 Å from urea molecule) becomes smaller by 10%. Moreover, the hydration structure around urea continues for a long time (16 ps), though the energetic relaxation time is very short.

The water–water pair potential near the hydrogen bonded equilibrium configuration

An analysis is presented of the contributions to the potential energy of interaction between water molecules near the hydrogen bonded equilibrium configuration. From this analysis we synthesize an effective potential that includes the influences of long-ranged and nonadditive interactions. The influence of the hydrogen bond bending on the anharmonic properties of ice I and on the spectral properties of amorphous solid water, the denser ice polymorphs, and liquid water are used to test the consistency of the effective potential. We also use that potential to estimate the quasistatic distribution of hydrogen bond angles in liquid water, which distribution is an important characteristic of the random network model of water.