Transferable model of water with variable molecular size

By decreasing the steepness of the repulsive wing in the intermolecular potential, one can extend the applicability of a water model to the high pressure region. Exploiting this trivial possibility, we published a polarizable model of water which provided good estimations not only of gas clusters, ambient liquid, hexagonal ice, but ice VII at very high pressures as well [A. Baranyai and P. Kiss, J. Chem. Phys. 133, 144109 (2010)]. This straightforward method works well provided the closest O-O distance is reasonably shorter in the high pressure phase than in hexagonal ice. If these O-O distances are close to each other and we fit the interactions to obtain an accurate picture of hexagonal ice, we underestimate the density of the high-pressure phases. This can be overcome if models use contracted molecules under high external pressure.In this paper we present a method, which is capable to describe the contraction of water molecules under high pressure by using two simple repulsion-attraction functions. These functions represent the dispersion interaction under low pressure and high pressure. The switch function varies between 0 and 1 and portions the two repulsions among the individual particles. The argument of the switch function is a virial-type expression, which can be interpreted as a net force compressing the molecule. We calculated the properties of gas clusters, densities, and internal energies of ambient water, hexagonal ice, ice III, ice VI, and ice VII phases and obtained excellent match of experimental data.     

Polarizable model of water with field-dependent polarization

The polarizable charge-on-spring model of water with three Gaussian charges developed by the present authors [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010)] was studied. We introduced an analytic function for the polarizability in terms of the local electric field. Following theoretical suggestions, the polarizability decreases from its experimental gas-phase value, in our approach, toward a high-field threshold. Using this modified polarizability, we reparameterized the model by calculating its dielectric constant and obtained good estimates of density and internal energy for ambient water, hexagonal ice, and water cluster properties. Mimicked by the new model, we studied liquid water under the impact of homogeneous static electric field in the rage of 0-2.5 V/?. Both the density and the average dipole moment increase with the strength of the electric field. However, the internal energy shows a minimum at ~0.35 V/?. At this field strength, the model starts ordering into a crystal structure. At higher fields the liquid forms a crystalline structure which is a special version of cubic ice.     

Classical interaction model for the water molecule

The authors propose a new classical model for the water molecule. The geometry of the molecule is built on the rigid TIP5P model and has the experimental gas phase dipole moment of water created by four equal point charges. The model preserves its rigidity but the size of the charges increases or decreases following the electric field created by the rest of the molecules. The polarization is expressed by an electric field dependent nonlinear polarization function. The increasing dipole of the molecule slightly increases the size of the water molecule expressed by the oxygen-centered sigma parameter of the Lennard-Jones interaction. After refining the adjustable parameters, the authors performed Monte Carlo simulations to check the ability of the new model in the ice, liquid, and gas phases. They determined the density and internal energy of several ice polymorphs, liquid water, and gaseous water and calculated the heat capacity, the isothermal compressibility, the isobar heat expansion coefficients, and the dielectric constant of ambient water. They also determined the pair-correlation functions of ambient water and calculated the energy of the water dimer. The accuracy of theirs results was satisfactory.     

Computer simulation study of water using a fluctuating charge model

Hydrogen bonding in small water clusters is studied through computer simulation methods using a sophisticated, empirical model of interaction developed by Ricket al (S W Rick, S J Stuart and B J Berne 1994J. Chem. Phys. 101 6141) and others. The model allows for the charges on the interacting sites to fluctuate as a function of time, depending on their local environment. The charge flow is driven by the difference in the electronegativity of the atoms within the water molecule, thus effectively mimicking the effects of polarization of the charge density. The potential model is thus transferable across all phases of water. Using this model, we have obtained the minimum energy structures of water clusters up to a size often. The cluster structures agree well with experimental data. In addition, we are able to distinctly identify the hydrogens that form hydrogen bonds based on their charges alone, a feature that is not possible in simulations using fixed charge models. We have also studied the structure of liquid water at ambient conditions using this fluctuating charge model.     

Study of lithium cation in water clusters: based on atom-bond electronegativity equalization method fused into molecular mechanics

We present a potential model for Li(+)-water clusters based on a combination of the atom-bond electronegativity equalization and molecular mechanics (ABEEM/MM) that is to take ABEEM charges of the cation and all atoms, bonds, and lone pairs of water molecules into the intermolecular electrostatic interaction term in molecular mechanics. The model allows point charges on cationic site and seven sites of an ABEEM-7P water molecule to fluctuate responding to the cluster geometry. The water molecules in the first sphere of Li(+) are strongly structured and there is obvious charge transfer between the cation and the water molecules; therefore, the charge constraint on the ionic cluster includes the charged constraint on the Li(+) and the first-shell water molecules and the charge neutrality constraint on each water molecule in the external hydration shells. The newly constructed potential model based on ABEEM/MM is first applied to ionic clusters and reproduces gas-phase state properties of Li(+)(H(2)O)(n) (n = 1-6 and 8) including optimized geometries, ABEEM charges, binding energies, frequencies, and so on, which are in fair agreement with those measured by available experiments and calculated by ab initio methods. Prospects and benefits introduced by this potential model are pointed out.     

Hydrogen bonding and induced dipole moments in water: predictions from the Gaussian charge polarizable model and Car-Parrinello molecular dynamics

We compare a new classical water model, which features Gaussian charges and polarizability (GCPM) with ab initio Car-Parrinello molecular dynamics (CPMD) simulations. We compare the total dipole moment, the total dipole moment distribution, and degree of hydrogen bonding at ambient to supercritical conditions. We also compared the total dipole moment calculated from both the electron density (partitioning the electron density among molecules based on a zero electron flux condition), and from the center of localized Wannier function centers (WFCs). Compared to CPMD, we found that GCPM overpredicts the dipole moment derived by partitioning the electron density and underpredicts that obtained from the WFCs, but exhibits similar trends and distribution of values. We also found that GCPM predicted similar degrees of hydrogen bonding compared to CPMD and has a similar structure.     

Density maximum and polarizable models of water

To estimate accurately the density of water over a wide range of temperatures with a density maximum at 4?°C is one of the most stringent tests of molecular models. The shape of the curve influences the ability to describe critical properties and to predict the freezing temperature. While it was demonstrated that with a proper parameter fit nonpolarizable models can approximate this behavior accurately, it is much more difficult to do this for polarizable models. We provide a short overview of ρ-T diagrams for existing models, then we give an explanation of this difficulty. We present a version of the BK model [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010); and ibid. 135, 234110 (2011)] which is capable to predict the density of water over a wide range of temperature. The BK model uses the charge-on-spring method with three Gaussian charges. Since the experimental dipole moment and the geometry is fixed, and the quadrupole moment is approximated by a least mean square procedure, parameters of the repulsion and dispersive attraction forces remained as free tools to match experimental properties. Relying on a simplified but plausible justification, the new version of the model uses repulsion and attraction as functions of the induced dipole moment of the molecule. The repulsive force increases, while the attractive force decreases with the size of the molecular dipole moment. At the same time dipole moment dependent dispersion forces are taking part in the polarization of the molecule. This scheme iterates well and, in addition to a reasonable density-temperature function, creates dipole distributions with accurate estimation of the dielectric constant of the liquid.     

X-ray scattering intensities of water at extreme pressure and temperature

We have calculated the coherent x-ray scattering intensity of several phases of water under high pressure using the ab initio density functional theory (DFT). Our calculations span the molecular liquid, ice VII, and superionic solid phases, including the recently predicted symmetrically hydrogen bonded region. We compute simulated spectra for ice VII and superionic water. We provide new atomic scattering form factors for water at extreme conditions, which take into account frequently neglected changes in ionic charge and electron delocalization. We show that our modified atomic form factors allow for a nearly exact comparison with the total x-ray scattering intensities calculated from DFT. Finally, we analyze the effect of their new form factors have on the determination of the oxygen-oxygen radial distribution function from experiment.     

Surface properties of the polarizable Baranyai-Kiss water model

The water surface properties using the Baranyai-Kiss (BK) model [A. Baranyai and P. T. Kiss, J. Chem. Phys. 133, 144109 (2010)] are studied by molecular dynamics simulation, and compared to popular rigid water potentials, namely to the extended simple point charge (SPC/E) and the transferable interaction potential with 4 points (TIP4P) models. The BK potential is a polarizable model of water with three Gaussian charges. The negative charge is connected to its field-free position by a classical harmonic spring, and mechanical equilibrium is established between this spring force and the force due to the charge distribution of the system. The aim of this study is, on the one hand, to test the surface properties of the new model, and on the other hand, to identify differences between the models listed above. The obtained results reveal that the BK model reproduces very well a number of properties corresponding to liquid-vapor equilibrium, such as the coexisting liquid and vapor densities, saturated vapor pressure or surface tension. Further, this model reproduces excellently the critical point of water even in comparison with a large number of widely used polarizable and nonpolarizable models. The structural properties of the liquid surface of BK water turns out to be very similar to that of the SPC/E model, while the surface of TIP4P water is found to be somewhat less ordered. This finding is related to the fact that the critical temperature of the TIP4P model is lower than that of either SPC/E or BK.     

A classical polarizable model for simulations of water and ice

We develop a classical rigid polarizable model of water for molecular simulations of water and ice. The model uses the Rowlinson five-site geometry: oxygen bearing the Lennard-Jones interaction and linearly polarizable point dipole, two positively charged hydrogens, and two massless negative charges placed symmetrically off oxygen so that the experimental dipole moment is reproduced. The target properties are the densities of water and ice Ih, diffusivity, enthalpies of fusion and vaporization, and the ice Ih melting point. The surface tension at lower temperatures is by 7% underestimated whereas the dielectric constant by 6% overestimated. Diffusivity and viscosity worsen at higher temperatures, although the Stokes radius is overestimated only by 2-7%. The ice Ih melting temperature is 260 K and the temperature of maximum density is 269 K. Rescaling the charges by a factor of 1.01 and Lennard-Jones energy by 1.0201 improves the melting point and energy-related quantities but shifts the agreement of kinetic properties to higher temperatures. For the model we propose abbreviation POL4D.     

A non-polarizable model of water that yields the dielectric constant and the density anomalies of the liquid: TIP4Q

A four-site rigid water model is presented, whose parameters are fitted to reproduce the experimental static dielectric constant at 298 K, the maximum density of liquid water and the equation of state at low pressures. The model has a positive charge on each of the three atomic nuclei and a negative charge located at the bisector of the HOH bending angle. This charge distribution allows increasing the molecular dipole moment relative to four-site models with only three charges and improves the liquid dielectric constant at different temperatures. Several other properties of the liquid and of ice Ih resulting from numerical simulations with the model are in good agreement with experimental values over a wide range of temperatures and pressures. Moreover, the model yields the minimum density of supercooled water at 190 K and the minimum thermal compressibility at 310 K, close to the experimental values. A discussion is presented on the structural changes of liquid water in the supercooled region where the derivative of density with respect to temperature is a maximum.     

Charge asymmetries in hydration of polar solutes

We study the solvation of polar molecules in water. The center of water's dipole moment is offset from its steric center. In common water models, the Lennard-Jones center is closer to the negatively charged oxygen than to the positively charged hydrogens. This asymmetry of water's charge sites leads to different hydration free energies of positive versus negative ions of the same size. Here, we explore these hydration effects for some hypothetical neutral solutes, and two real solutes, with molecular dynamics simulations using several different water models. We find that, like ions, polar solutes are solvated differently in water depending on the sign of the partial charges. Solutes having a large negative charge balancing diffuse positive charges are preferentially solvated relative to those having a large positive charge balancing diffuse negative charges. Asymmetries in hydration free energies can be as large as 10 kcal/mol for neutral benzene-sized solutes. These asymmetries are mainly enthalpic, arising primarily from the first solvation shell water structure. Such effects are not readily captured by implicit solvent models, which respond symmetrically with respect to charge.     

Semiempirical self-consistent polarization description of bulk water, the liquid-vapor interface, and cubic ice

We have applied an efficient electronic structure approach, the semiempirical self-consistent polarization neglect of diatomic differential overlap (SCP-NDDO) method, previously parametrized to reproduce properties of water clusters by Chang, Schenter, and Garrett [ J. Chem. Phys. 2008 , 128 , 164111 ] and now implemented in the CP2K package, to model ambient liquid water at 300 K (both the bulk and the liquid-vapor interface) and cubic ice at 15 and 250 K. The SCP-NDDO potential retains its transferability and good performance across the full range of conditions encountered in the clusters and the bulk phases of water. In particular, we obtain good results for the density, radial distribution functions, enthalpy of vaporization, self-diffusion coefficient, molecular dipole moment distribution, and hydrogen bond populations, in comparison to experimental measurements.     

Prelude to molecular dynamics: Topography-driven gaussian charge models

A new strategy to develop Gaussian charge models (GCMs) for molecules like ammonia, water, ethene, hydrogen sulfide, formaldehyde and benzene is presented. These molecular models comprising of positive point charges and negative Gaussian charge distributions (GCDs), which represent nuclei and continuous electron charge distribution, are found to correctly represent the ab initio Molecular Electrostatic Potential (MESP) and reproduce its essential topographical features of corresponding molecules. The models use optimized parameters: positive charges at nuclei, negative charges on GCDs, Gaussian exponent and centers. The Potential Energy Surface (PES) of water dimer has been explored using water GCMs. A good agreement has been found between PES obtained using GCMs and wave function. The Gaussian models correctly predict structure of benzene-water complex. It is thus recommended to use GCMs for molecular dynamic simulations.     

Electrostatic water models

The possiblity of creating a useful model for the electrostatic potential around the water molecule is examined from two different starting points. It is suggested that the simplest and most accurate model will have three point charges on the nuclei together with a diffuse Gaussian charge centered near the nuclear center of the charge. This will represent the lone-pair cloud, include the penetration effects, and improve the representation of the electron density. The alternative use of atomic dipoles is explored but seen to be less effective.     

Ab initio parameterization of an all-atom polarizable and dissociable force field for water

A novel all-atom, dissociative, and polarizable force field for water is presented. The force field is parameterized based on forces, stresses, and energies obtained form ab initio calculations of liquid water at ambient conditions. The accuracy of the force field is tested by calculating structural and dynamical properties of liquid water and the energetics of small water clusters. The transferability of the force field to dissociated states is studied by considering the solvation of a proton and the ionization of water at extreme conditions of pressure and temperature. In the case of the solvated proton, the force field properly describes the presence of both Eigen and Zundel configurations. In the case of the pressure-induced ice VIII/ice X transition and the temperature-induced transition to a superionic phase, the force field is found to describe accurately the proton symmetrization and the melting of the proton sublattice, respectively.     

Electrical processes upon the evaporation and condensation of water and ice

A mathematical model of electrical processes that take place upon the evaporation of water and sublimation of ice, as well as the condensation growth of water and ice phases from vapor, is proposed. The transfer of the main charge carriers, such as (i) protons and hydroxide ions (in ice, water, and vapor and (ii) orientational defects (in ice and water) is taken into account. Upon the evaporation of water and the sublimation of ice, the first carriers are accumulated before the phase front and cause positive charges in the surface of the water and ice, while the second carriers are depleted (their concentration becomes lower than the thermodynamic value) and impart a negative charge to water and ice. The contribution of protons and hydroxide ions dominates at a low rate of evaporation. In the condensation of vapor and relevant growth of water and ice phases, the polarity of surface charge is opposite to that observed upon evaporation. The values of interfacial current and signs of phase charges upon sublimation, evaporation, and condensation that are predicted in the model comply with experimental data.