Computer simulation of two new solid phases of water: Ice XIII and ice XIV

NpT Monte Carlo simulations have been performed for two recently discovered solid phases of water which have been denoted as ice XIII and ice XIV C. G. Salzmann et al. [Science311, 1758 (2006)]. Several potential models of water were considered, namely, the traditional SPC/E, TIP4P, and TIP5P and the more recent TIP5P-E, TIP4P-Ew, TIP4P/Ice, and TIP4P/2005 models. Significant differences in density and oxygen-oxygen radial distribution functions are found between the predictions of the SPC/E, TIP5P, and the models of the TIP4P family. The models TIP4P/Ice and TIP4P/2005 provide the best estimates of the density.     

The melting temperature of the most common models of water

The melting temperature of ice I(h) for several commonly used models of water (SPC, SPC/E,TIP3P,TIP4P, TIP4P/Ew, and TIP5P) is obtained from computer simulations at p = 1 bar. Since the melting temperature of ice I(h) for the TIP4P model is now known [E. Sanz, C. Vega, J. L. F. Abascal, and L. G. MacDowell, Phys. Rev. Lett. 92, 255701 (2004)], it is possible to use the Gibbs-Duhem methodology [D. Kofke, J. Chem. Phys. 98, 4149 (1993)] to evaluate the melting temperature of ice I(h) for other potential models of water. We have found that the melting temperatures of ice I(h) for SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P models are T = 190 K, 215 K, 146 K, 232 K, 245 K, and 274 K, respectively. The relative stability of ice I(h) with respect to ice II for these models has also been considered. It turns out that for SPC, SPC/E, TIP3P, and TIP5P the stable phase at the normal melting point is ice II (so that ice I(h) is not a thermodynamically stable phase for these models). For TIP4P and TIP4P/Ew, ice I(h) is the stable solid phase at the standard melting point. The location of the negative charge along the H-O-H bisector appears as a critical factor in the determination of the relative stability between the I(h) and II ice forms. The methodology proposed in this paper can be used to investigate the effect upon a coexistence line due to a change in the potential parameters.     

Melting points and thermal expansivities of proton-disordered hexagonal ice with several model potentials

A method of free energy calculation is proposed, which enables to cover a wide range of pressure and temperature. The free energies of proton-disordered hexagonal ice (ice Ih) and liquid water are calculated for the TIP4P [J. Chem. Phys. 79, 926 (1983)] model and the TIP5P [J. Chem. Phys. 112, 8910 (2000)] model. From the calculated free energy curves, we determine the melting point of the proton-disordered hexagonal ice at 0.1 MPa (atmospheric pressure), 50 MPa, 100 MPa, and 200 MPa. The melting temperatures at atmospheric pressure for the TIP4P ice and the TIP5P ice are found to be about T(m)=229 K and T(m)=268 K, respectively. The melting temperatures decrease as the pressure is increased, a feature consistent with the pressure dependence of the melting point for realistic proton-disordered hexagonal ice. We also calculate the thermal expansivity of the model ices. Negative thermal expansivity is observed at the low temperature region for the TIP4P ice, but not for the TIP5P ice at the ambient pressure.     

The thickness of a liquid layer on the free surface of ice as obtained from computer simulation

Molecular dynamic simulations were performed for ice I(h) with a free surface by using four water models, SPC/E, TIP4P, TIP4P/Ice, and TIP4P/2005. The behavior of the basal plane, the primary prismatic plane, and of the secondary prismatic plane when exposed to vacuum was analyzed. We observe the formation of a thin liquid layer at the ice surface at temperatures below the melting point for all models and the three planes considered. For a given plane it was found that the thickness of a liquid layer was similar for different water models, when the comparison is made at the same undercooling with respect to the melting point of the model. The liquid layer thickness is found to increase with temperature. For a fixed temperature it was found that the thickness of the liquid layer decreases in the following order: the basal plane, the primary prismatic plane, and the secondary prismatic plane. For the TIP4P/Ice model, a model reproducing the experimental value of the melting temperature of ice, the first clear indication of the formation of a liquid layer, appears at about -100 degrees C for the basal plane, at about -80 degrees C for the primary prismatic plane, and at about -70 degrees C for the secondary prismatic plane.     

Relation between the melting temperature and the temperature of maximum density for the most common models of water

Water exhibits a maximum in density at normal pressure at 4 degrees above its melting point. The reproduction of this maximum is a stringent test for potential models used commonly in simulations of water. The relation between the melting temperature and the temperature of maximum density for these potential models is unknown mainly due to our ignorance about the melting temperature of these models. Recently we have determined the melting temperature of ice I(h) for several commonly used models of water (SPC, SPC/E, TIP3P, TIP4P, TIP4P/Ew, and TIP5P). In this work we locate the temperature of maximum density for these models. In this way the relative location of the temperature of maximum density with respect to the melting temperature is established. For SPC, SPC/E, TIP3P, TIP4P, and TIP4P/Ew the maximum in density occurs at about 21-37 K above the melting temperature. In all these models the negative charge is located either on the oxygen itself or on a point along the H-O-H bisector. For the TIP5P and TIP5P-E models the maximum in density occurs at about 11 K above the melting temperature. The location of the negative charge appears as a geometrical crucial factor to the relative position of the temperature of maximum density with respect to the melting temperature.     

The melting point of ice Ih for common water models calculated from direct coexistence of the solid-liquid interface

In this work we present an implementation for the calculation of the melting point of ice I(h) from direct coexistence of the solid-liquid interface. We use molecular dynamics simulations of boxes containing liquid water and ice in contact. The implementation is based on the analysis of the evolution of the total energy along NpT simulations at different temperatures. We report the calculation of the melting point of ice I(h) at 1 bar for seven water models: SPC/E, TIP4P, TIP4P-Ew, TIP4P/ice, TIP4P/2005, TIP5P, and TIP5P-E. The results for the melting temperature from the direct coexistence simulations of this work are in agreement (within the statistical uncertainty) with those obtained previously by us from free energy calculations. By taking into account the results of this work and those of our free energy calculations, recommended values of the melting point of ice I(h) at 1 bar for the above mentioned water models are provided.     

Prediction of self-diffusion coefficient and shear viscosity of water and its binary mixtures with methanol and ethanol by molecular simulation

Density, self-diffusion coefficient, and shear viscosity of pure liquid water are predicted for temperatures between 280 and 373 K by molecular dynamics simulation and the Green-Kubo method. Four different rigid nonpolarizable water models are assessed: SPC, SPC/E, TIP4P, and TIP4P/2005. The pressure dependence of the self-diffusion coefficient and the shear viscosity for pure liquid water is also calculated and the anomalous behavior of these properties is qualitatively well predicted. Furthermore, transport properties as well as excess volume and excess enthalpy of aqueous binary mixtures containing methanol or ethanol, based on the SPC/E and TIP4P/2005 water models, are calculated. Under the tested conditions, the TIP4P/2005 model gives the best quantitative and qualitative agreement with experiments for the regarded transport properties. The deviations from experimental data are of 5% to 15% for pure liquid water and 5% to 20% for the water + alcohol mixtures. Moreover, the center of mass power spectrum of water as well as the investigated mixtures are analyzed and the hydrogen-bonding structure is discussed for different states.     

Computer simulation study of metastable ice VII and amorphous phases obtained by its melting

Molecular dynamics simulations of metastable ice VII and cubic ice Ic are carried out in order to examine (1) the ability of commonly used water interaction potentials to reproduce the properties of ices, and (2) the possibility of generating low-density amorphous (LDA) structures by heating ice VII, which is known to transform to LDA at approximately 135 K at normal pressure [S. Klotz, J. M. Besson, G. Hamel, R. J. Nelmes, J. S. Loveday, and W. G. Marshall, Nature (London) 398, 681 (1999)]. We test four simple empirical interaction potentials of water: TIP4P [W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 (1983)], SPC/E [H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma, J. Phys. Chem. B 91, 6269 (1987)], TIP5P [M. W. Mahoney and W. L. Jorgensen, J. Chem. Phys. 112, 8910 (2000)], and ST2 [F. H. Stillinger and A. Rahman, J. Chem. Phys. 60, 1545 (1974)]. We have found that TIP5P ice VII melts at 210 K, TIP4P at 90 K, and SPC/E at 70 K. Only TIP5P water after transition has a structure similar to that of LDA. TIP4P and SPC/E have almost identical structures, dissimilar to any known water or amorphous phases, but upon heating both slowly evolve towards LDA-like structure. ST2 ice VII is remarkably stable up to 430 K. TIP4P and SPC/E predict correctly the cubic ice collapse into a high-density amorphous ice (HDA) at approximately 1 GPa whereas TIP5P remains stable up to approximately 5 GPa. The densities of the simulated ice phases differ significantly, depending on the potential used, and are generally higher than experimental values. The importance of proper treatment of long-range electrostatic interactions is also discussed.     

Dynamics of TIP5P and TIP4P/ice potentials

The dynamics of a thin film of ice Ih deposited on MgO (001) is studied through molecular dynamics simulations performed with two new potential models of ice. This system is chosen because it is possible to compare the results of the simulations to incoherent neutron quasielastic scattering experiments performed few years ago and to previous molecular dynamics simulations using the TIP4P potential model. The present simulations are performed to determine the evolution of the translational and orientational order parameters of the ice film upon temperature increase in the 250-280 K range. They are also used to calculate the translational and orientational diffusion coefficients of the water molecules in the supported film as a function of the temperature. When using the TIP5P potential, the present results show a better agreement with experimental data than those calculated with the TIP4P potential, especially regarding the temperature above which significant changes are obtained in the dynamics of the water film. Similar conclusions are obtained when using the TIP4P/ice potential, although this latter potential clearly underestimates the translational diffusion coefficients.     

Melting and superheating of sI methane hydrate: molecular dynamics study

Melting and decay of the superheated sI methane structure are studied using molecular dynamics simulation. The melting curve is calculated by the direct coexistence simulations in a wide range of pressures up to 5000 bar for the SPC/E, TIP4P/2005 and TIP4P/Ice water models and the united-atom model for methane. We locate the kinetic stability boundary of the superheated metastable sI structure that is found to be surprisingly high comparing with the predictions based on the classical nucleation theory.     

Molecular dynamics simulations of ice nucleation by electric fields

Molecular dynamics simulations are used to investigate heterogeneous ice nucleation in model systems where an electric field acts on water molecules within 10-20 ? of a surface. Two different water models (the six-site and TIP4P/Ice models) are considered, and in both cases, it is shown that a surface field can serve as a very effective ice nucleation catalyst in supercooled water. Ice with a ferroelectric cubic structure nucleates near the surface, and dipole disordered cubic ice grows outward from the surface layer. We examine the influences of temperature and two important field parameters, the field strength and distance from the surface over which it acts, on the ice nucleation process. For the six-site model, the highest temperature where we observe field-induced ice nucleation is 280 K, and for TIP4P/Ice 270 K (note that the estimated normal freezing points of the six-site and TIP4P/Ice models are ～289 and ～270 K, respectively). The minimum electric field strength required to nucleate ice depends a little on how far the field extends from the surface. If it extends 20 ?, then a field strength of 1.5 × 10(9) V/m is effective for both models. If the field extent is 10 ?, then stronger fields are required (2.5 × 10(9) V/m for TIP4P/Ice and 3.5 × 10(9) V/m for the six-site model). Our results demonstrate that fields of realistic strength, that act only over a narrow surface region, can effectively nucleate ice at temperatures not far below the freezing point. This further supports the possibility that local electric fields can be a significant factor influencing heterogeneous ice nucleation in physical situations. We would expect this to be especially relevant for ice nuclei with very rough surfaces where one would expect local fields of varying strength and direction.     

A potential model for the study of ices and amorphous water: TIP4P/Ice

The ability of several water models to predict the properties of ices is discussed. The emphasis is put on the results for the densities and the coexistence curves between the different ice forms. It is concluded that none of the most commonly used rigid models is satisfactory. A new model specifically designed to cope with solid-phase properties is proposed. The parameters have been obtained by fitting the equation of state and selected points of the melting lines and of the coexistence lines involving different ice forms. The phase diagram is then calculated for the new potential. The predicted melting temperature of hexagonal ice (Ih) at 1 bar is 272.2 K. This excellent value does not imply a deterioration of the rest of the properties. In fact, the predictions for both the densities and the coexistence curves are better than for TIP4P, which previously yielded the best estimations of the ice properties.     

The importance of polarizability in the modeling of solubility: quantifying the effect of solute polarizability on the solubility of small nonpolar solutes in popular models of water

In recent work by Paschek [J. Chem. Phys. 120, 6674 (2004)] and others [see H. Docherty et al., J. Chem. Phys. 125, 074510 (2006) for a review] it has been suggested that, when coupled to a simple Lennard-Jones model for various small nonpolar solute molecules, the most common models of water (e.g., SPC/E and TIP4P) fail to reproduce quantitatively the solubility of small nonpolar solute molecules in water due in part to failing to account for polarization of the solute molecule. Given the importance of such systems as test-case prototype models of the solubility of proteins and biomolecules, in this work, we investigate the impact of using a polarizable solute model with the SPC/E, TIP3P, TIP4P, TIP4P-Ew, and TIP4P/2005 rigid water models. Specifically we consider Ne, Ar, Kr, Xe, and methane as solutes. In all cases we observe that the use of a polarizable solute improves agreement between experiment and simulations, with the best agreement seen for the largest solutes, Kr, CH(4), and Xe and the modern reparametrizations of the TIP4P model, i.e., the TIP4P-Ew and TIP4P/2005 models.     

Simulated surface tensions of common water models

Initial simulated values of the surface tension for the SPC/E water model have indicated excellent agreement with experiment. More recently, differing values have been obtained which are significantly lower than previous estimates. Here, we attempt to explain the differences between the previous studies and show that a variety of simulation conditions can affect the final surface tension values. Consistent values for the surface tensions of six common fixed charge water models (TIP3P, SPC, SPC/E, TIP4P, TIP5P, and TIP6P) are then determined for four temperatures between 275 and 350 K. The SPC/E and TIP6P models provide the best agreement with experiment.     

Efficient sampling of ice structures by electrostatic switching

An electrostatic switching procedure is introduced that enables the systematic generation of high-quality ice configurations at various temperatures. Proton disordered ice Ih configurations were generated for the TIP4P water model at temperatures from 50 to 240 K, for the SPC/Fw water model from 100 to 240 K, and for the DC97 water model at 240 K. The resulting configurations were found to properly sample the canonical ensemble. The dielectric constant of ice Ih was determined from the net dipole fluctuation of the ice configurations. The calculated dielectric constant compares favorably with the study by Rick and Haymet [J. Chem. Phys. 2003, 118, 9291]. However, our method gives smaller error bars, especially at lower temperatures. At temperatures above 200 K, a type of hydrogen-bond defect is identified that cannot be categorized as a D or L type defect.