Simulations of H2O Solid, Liquid, and Clusters, with an Emphasis on Ferroelectric Ordering Transition in Hexagonal Ice

Three-phase equilibria of hydrates from computer simulation. II. Finite-size effects in the carbon dioxide hydrate

In this work, the effects of finite size on the determination of the three-phase coexistence temperature (T3) of the carbon dioxide (CO2) hydrate have been studied by molecular dynamic simulations and using the direct coexistence technique. According to this technique, the three phases involved (hydrate-aqueous solution-liquid CO2) are placed together in the same simulation box. By varying the number of molecules of each phase, it is possible to analyze the effect of simulation size and stoichiometry on the T3 determination. In this work, we have determined the T3 value at 8 different pressures (from 100 to 6000 bar) and using 6 different simulation boxes with different numbers of molecules and sizes. In two of these configurations, the ratio of the number of water and CO2 molecules in the aqueous solution and the liquid CO2 phase is the same as in the hydrate (stoichiometric configuration). In both stoichiometric configurations, the formation of a liquid drop of CO2 in the aqueous phase is observed. This drop, which has a cylindrical geometry, increases the amount of CO2 available in the aqueous solution and can in some cases lead to the crystallization of the hydrate at temperatures above T3, overestimating the T3 value obtained from direct coexistence simulations. The simulation results obtained for the CO2 hydrate confirm the sensitivity of T3 depending on the size and composition of the system, explaining the discrepancies observed in the original work by Míguez et al. [J. Chem Phys. 142, 124505 (2015)]. Non-stoichiometric configurations with larger unit cells show a convergence of T3 values, suggesting that finite-size effects for these system sizes, regardless of drop formation, can be safely neglected. The results obtained in this work highlight that the choice of a correct initial configuration is essential to accurately estimate the three-phase coexistence temperature of hydrates by direct coexistence simulations.

Three-phase equilibria of hydrates from computer simulation. I. Finite-size effects in the methane hydrate

Clathrate hydrates are vital in energy research and environmental applications. Understanding their stability is crucial for harnessing their potential. In this work, we employ direct coexistence simulations to study finite-size effects in the determination of the three-phase equilibrium temperature (T3) for methane hydrates. Two popular water models, TIP4P/Ice and TIP4P/2005, are employed, exploring various system sizes by varying the number of molecules in the hydrate, liquid, and gas phases. The results reveal that finite-size effects play a crucial role in determining T3. The study includes nine configurations with varying system sizes, demonstrating that smaller systems, particularly those leading to stoichiometric conditions and bubble formation, may yield inaccurate T3 values. The emergence of methane bubbles within the liquid phase, observed in smaller configurations, significantly influences the behavior of the system and can lead to erroneous temperature estimations. Our findings reveal finite-size effects on the calculation of T3 by direct coexistence simulations and clarify the system size convergence for both models, shedding light on discrepancies found in the literature. The results contribute to a deeper understanding of the phase equilibrium of gas hydrates and offer valuable information for future research in this field.

Estimation of the ice melting point in molecular dynamics simulations based on the finite-size effects

Predicting the ice melting point using molecular dynamics (MD) simulations is nontrivial due to uncertainty associated with the stochastic nature of the simulation and effect of finite domain sizes on the simulated ice-water phase transition. We developed a method based on the percolation theory to make use of the finite size effects to allow determination of a unique critical phase transition temperature as the melting point. The method involves construction of melting/freezing probability curves from multiple simulations with varying temperatures for different domain sizes. While the domain sizes affect the apparent melting/freezing probability and hence generate different curves with a wider probability distribution for a smaller size, the intersection of these curves is unique and locates the melting point. Based on MD simulations using the Tip4p/Ice water model, we tested and demonstrated the effectiveness of this method in locating the critical ice-water phase transition at a melting temperature of 268.78 K. Our analysis also showed that the apparent melting probability at this critical point is ∼0.69, not 0.5 assumed in the ad hoc method used previously. Our method, making no assumption about the system size, may provide a generic framework for analyzing phase transitions influenced by the finite size effects in MD simulations.

Inhibition of Defect-Induced Ice Nucleation, Propagation, and Adhesion by Bioinspired Self-Healing Anti-Icing Coatings

Anti-icing coatings on outdoor infrastructures inevitably suffer from mechanical injuries in numerous icing scenarios such as hailstorms, sandstorms, impacts of foreign objects, and icing-deicing cycles. Herein, the mechanisms of surface-defect-induced icing are clarified. At the defects, water molecules exhibit stronger adsorption and the heat transfer rate increases, accelerating the condensation of water vapor as well as ice nucleation and propagation. Moreover, the ice-defect interlocking structure increases the ice adhesion strength. Thus, a self-healing (at -20 °C) antifreeze-protein (AFP)-inspired anti-icing coating is developed. The coating is based on a design that mimics the ice-binding and non-ice-binding sites in AFPs. It enables the coating to markedly inhibit ice nucleation (nucleation temperature < -29.4 °C), prevent ice propagation (propagation rate < 0.00048 cm²/s), and reduce ice adhesion on the surface (adhesion strength < 38.9 kPa). More importantly, the coating can also autonomously self-heal at -20 °C, as a result of multiple dynamic bonds in its structure, to inhibit defect-induced icing processes. The healed coating sustains high anti-icing and deicing performance even under various extreme conditions. This work reveals the in-depth mechanism of defect-induced ice formation as well as adhesion, and proposes a self-healing anti-icing coating for outdoor infrastructures.

Homogeneous nucleation rate of methane hydrate formation under experimental conditions from seeding simulations

In this work, we shall estimate via computer simulations the homogeneous nucleation rate for the methane hydrate at 400 bars for a supercooling of about 35 K. The TIP4P/ICE model and a Lennard-Jones center were used for water and methane, respectively. To estimate the nucleation rate, the seeding technique was employed. Clusters of the methane hydrate of different sizes were inserted into the aqueous phase of a two-phase gas-liquid equilibrium system at 260 K and 400 bars. Using these systems, we determined the size at which the cluster of the hydrate is critical (i.e., it has 50% probability of either growing or melting). Since nucleation rates estimated from the seeding technique are sensitive to the choice of the order parameter used to determine the size of the cluster of the solid, we considered several possibilities. We performed brute force simulations of an aqueous solution of methane in water in which the concentration of methane was several times higher than the equilibrium concentration (i.e., the solution was supersaturated). From brute force runs, we infer the value of the nucleation rate for this system rigorously. Subsequently, seeding runs were carried out for this system, and it was found that only two of the considered order parameters were able to reproduce the value of the nucleation rate obtained from brute force simulations. By using these two order parameters, we estimated the nucleation rate under experimental conditions (400 bars and 260 K) to be of the order of log₁₀ (J/(m³ s)) = -7(5).

Phase diagrams-Why they matter and how to predict them

Understanding the thermodynamic stability and metastability of materials can help us to, for example, gauge whether crystalline polymorphs in pharmaceutical formulations are likely to be durable. It can also help us to design experimental routes to novel phases with potentially interesting properties. In this Perspective, we provide an overview of how thermodynamic phase behavior can be quantified both in computer simulations and machine-learning approaches to determine phase diagrams, as well as combinations of the two. We review the basic workflow of free-energy computations for condensed phases, including some practical implementation advice, ranging from the Frenkel-Ladd approach to thermodynamic integration and to direct-coexistence simulations. We illustrate the applications of such methods on a range of systems from materials chemistry to biological phase separation. Finally, we outline some challenges, questions, and practical applications of phase-diagram determination which we believe are likely to be possible to address in the near future using such state-of-the-art free-energy calculations, which may provide fundamental insight into separation processes using multicomponent solvents.

Solubility of Methane in Water: Some Useful Results for Hydrate Nucleation

In this paper, the solubility of methane in water along the 400 bar isobar is determined by computer simulations using the TIP4P/Ice force field for water and a simple LJ model for methane. In particular, the solubility of methane in water when in contact with the gas phase and the solubility of methane in water when in contact with the hydrate has been determined. The solubility of methane in a gas-liquid system decreases as temperature increases. The solubility of methane in a hydrate-liquid system increases with temperature. The two curves intersect at a certain temperature that determines the triple point T₃ at a certain pressure. We also determined T₃ by the three-phase direct coexistence method. The results of both methods agree, and we suggest 295(2) K as the value of T₃ for this system. We also analyzed the impact of curvature on the solubility of methane in water. We found that the presence of curvature increases the solubility in both the gas-liquid and hydrate-liquid systems. The change in chemical potential for the formation of hydrate is evaluated along the isobar using two different thermodynamic routes, obtaining good agreement between them. It is shown that the driving force for hydrate nucleation under experimental conditions is higher than that for the formation of pure ice when compared at the same supercooling. We also show that supersaturation (i.e., concentrations above those of the planar interface) increases the driving force for nucleation dramatically. The effect of bubbles can be equivalent to that of an additional supercooling of about 20 K. Having highly supersaturated homogeneous solutions makes possible the spontaneous formation of the hydrate at temperatures as high as 285 K (i.e., 10K below T₃). The crucial role of the concentration of methane for hydrate formation is clearly revealed. Nucleation of the hydrate can be either impossible or easy and fast depending on the concentration of methane which seems to play the leading role in the understanding of the kinetics of hydrate formation.

The liquidus temperature curve of aqueous methanol mixtures: a numerical simulation study

The liquidus temperature curve that characterizes the boundary between the liquid methanol/water mixture and its coexistence with ice Ih is determined using the direct-coexistence method. Several methanol concentrations and pressures of 0.1 MPa, 50 MPa, and 100 MPa are considered. In this study, we used the TIP4P/Ice model for water and two different models for methanol: OPLS and OPLS/2016, using the geometric rule for the Lennard-Jones cross interactions. We compared our simulation results with available experimental data and found that this combination of models reproduces reasonably well the liquidus curve for methanol mole fractions up to xm=0.3 at p=0.1 MPa. The freezing point depression of these mixtures is calculated and compared to experimental results. We also analyzed the effect of pressure on the liquidus curve, and we found that both models also reproduce qualitatively well the experimental decreasing of the liquidus temperatures as the pressure increases.

Melting points of water models: Current situation

By using the direct coexistence method, we have calculated the melting points of ice I h at normal pressure for three recently proposed water models, namely, TIP3P-FB, TIP4P-FB, and TIP4P-D. We obtained T m = 216 K for TIP3P-FB, T m = 242 K for TIP4P-FB, and T m = 247 K for TIP4P-D. We revisited the melting point of TIP4P/2005 and TIP5P obtaining T m = 250 and 274 K, respectively. We summarize the current situation of the melting point of ice I h for a number of water models and conclude that no model is yet able to simultaneously reproduce the melting temperature of ice I h and the temperature of the maximum in density at room pressure. This probably points toward our both still incomplete knowledge of the potential energy surface of water and the necessity of incorporating nuclear quantum effects to describe both properties simultaneously.

Adsorption of C2-C5 alcohols on ice. A grand canonical Monte Carlo simulation study

In this paper, we report Grand Canonical Monte Carlo simulations performed to characterize the adsorption of four linear alcohol molecules, comprising between 2 and 5 carbon atoms (namely, ethanol, n-propanol, n-butanol, and n-pentanol) on crystalline ice in a temperature range typical of the Earth's troposphere.The adsorption details analysed at 228 K show that, at low coverage of the ice surface, the polar head of the adsorbed molecules tend to optimize its hydrogen bonding with the surrounding water, whereas the aliphatic chain lie more or less parallel to the ice surface. With increasing coverage, the lateral interactions between the adsorbed alcohol molecules lead to the reorientation of the aliphatic chains which tend to become perpendicular to the surface, the adsorbed molecules pointing thus their terminal methyl group up to the gas phase. When compared to the experimental data, the simulated and measured isotherms show a very good agreement, although a small temperature shift between simulations and experiments could be inferred from simulations at various temperatures. In addition, this agreement appears to be better for ethanol and n-propanol than for n-butanol and n-pentanol, especially at the highest pressures investigated, pointing to a possible slight underestimation of the lateral interactions between the largest alcohol molecules by the interaction potential model used. Nevertheless, the global accuracy of the approach used, as tested in tropospheric conditions, opens the way for its use in modeling studies also relevant to another (e.g., astrophysical) context.

Freezing point depression of salt aqueous solutions using the Madrid-2019 model

Salt aqueous solutions are relevant in many fields, ranging from biological systems to seawater. Thus, the availability of a force-field that is able to reproduce the thermodynamic and dynamic behavior of salt aqueous solutions would be of great interest. Unfortunately, this has been proven challenging, and most of the existing force-fields fail to reproduce much of their behavior. In particular, the diffusion of water or the salt solubility are often not well reproduced by most of the existing force-fields. Recently, the Madrid-2019 model was proposed, and it was shown that this force-field, which uses the TIP4P/2005 model for water and non-integer charges for the ions, provides a good description of a large number of properties, including the solution densities, viscosities, and the diffusion of water. In this work, we assess the performance of this force-field on the evaluation of the freezing point depression. Although the freezing point depression is a colligative property that at low salt concentrations depends solely on properties of pure water, a good model for the electrolytes is needed to accurately predict the freezing point depression at moderate and high salt concentrations. The coexistence line between ice and several salt aqueous solutions (NaCl, KCl, LiCl, MgCl₂, and Li₂SO₄) up to the eutectic point is estimated from direct coexistence molecular dynamics simulations. Our results show that this force-field reproduces fairly well the experimentally measured freezing point depression with respect to pure water freezing for all the salts and at all the compositions considered.

On the anomalous homogeneity of hydrogen-disordered ice and its origin

Pauling's successful estimation of the residual entropy of hydrogen-disordered ice was based on the homogeneity of the binding energy of individual water molecules in ice. However, it has not been explained why the binding energies are homogeneous although the pair interaction energy of hydrogen-bonded dimers distributes widely. Here, we provide a rationale for this phenomenon. The topological constraints imposed by the ice rules, in which water molecules form directed cyclic paths of hydrogen bonds, cancel out the variability of local interactions. We also show that the cancellation mechanism does not work due to some imperfect cyclic paths on the surface of ice. Such water molecules do not enjoy homogeneity in the bulk state and suffer from a wide spectrum in the binding energy.

Spontaneous NaCl-doped ices Ih, Ic, III, V and VI. Understanding the mechanism of ion inclusion and its dependence on the crystalline structure of ice

Direct coexistence simulations on a microsecond time scale have been performed for different types of ice (I_h, I_c, III, V, and VI) in contact with a NaCl aqueous solution at different pressures. In line with the previous results obtained for ice I_h [Conde et al., Phys. Chem. Chem. Phys., 2017, 19, 9566-9574], our results reveal the spontaneous growth of a new ice doped phase and the formation of a brine rejection phase in all ices studied. However, both the preferential incorporation of ions into the ice lattice and the inclusion mechanisms depend on the crystalline structure of each ice. This work shows the inclusion of Cl^- and Na⁺ ions in ice from salt using molecular dynamics simulation, in agreement with the experimental evidence found in the literature. The model used for water is TIP4P/2005. For NaCl we employ a set of potential parameters that uses unit charges for the ions.

Phase Diagram of a Deep Potential Water Model

Using the Deep Potential methodology, we construct a model that reproduces accurately the potential energy surface of the SCAN approximation of density functional theory for water, from low temperature and pressure to about 2400 K and 50 GPa, excluding the vapor stability region. The computational efficiency of the model makes it possible to predict its phase diagram using molecular dynamics. Satisfactory overall agreement with experimental results is obtained. The fluid phases, molecular and ionic, and all the stable ice polymorphs, ordered and disordered, are predicted correctly, with the exception of ice III and XV that are stable in experiments, but metastable in the model. The evolution of the atomic dynamics upon heating, as ice VII transforms first into ice VII^{''} and then into an ionic fluid, reveals that molecular dissociation and breaking of the ice rules coexist with strong covalent fluctuations, explaining why only partial ionization was inferred in experiments.

Novel Algorithm to Generate Hydrogen-Disordered Ice Structures

We propose an efficient algorithm for generating hydrogen-disordered ice networks utilizing graph theory and the topological characteristic of the network. The computational efficiency with the new algorithm is much higher than the conventional ones developed by Rahman and Stillinger and Buch et al. The difference in the computational time between our algorithm and either of the two conventional ones increases with increasing the system size.

Effect of Water Content on Thermodynamic Properties of Compressed Hydrogen

Force field-based molecular simulations were used to calculate thermal expansivities, heat capacities, and Joule-Thomson coefficients of binary (standard) hydrogen-water mixtures for temperatures between 366.15 and 423.15 K and pressures between 50 and 1000 bar. The mole fraction of water in saturated hydrogen-water mixtures in the gas phase ranges from 0.004 to 0.138. The same properties were calculated for pure hydrogen at 323.15 K and pressures between 100 and 1000 bar. Simulations were performed using the TIP3P and a modified TIP4P force field for water and the Marx, Vrabec, Cracknell, Buch, and Hirschfelder force fields for hydrogen. The vapor-liquid equilibria of hydrogen-water mixtures were calculated along the melting line of ice Ih, corresponding to temperatures between 264.21 and 272.4 K, using the TIP3P force field for water and the Marx force field for hydrogen. In this temperature range, the solubilities and the chemical potentials of hydrogen and water were obtained. Based on the computed solubility data of hydrogen in water, the freezing-point depression of water was computed ranging from 264.21 to 272.4 K. The modified TIP4P and Marx force fields were used to improve the solubility calculations of hydrogen-water mixtures reported in our previous study [Rahbari A.;J. Chem. Eng. Data2019, 64, 4103-4115] for temperatures between 323 and 423 K and pressures ranging from 100 to 1000 bar. The chemical potentials of ice Ih were calculated as a function of pressure between 100 and 1000 bar, along the melting line for temperatures between 264.21 and 272.4 K, using the IAPWS equation of state for ice Ih. We show that at low pressures, the presence of water has a large effect on the thermodynamic properties of compressed hydrogen. Our conclusions may have consequences for the energetics of a hydrogen refueling station using electrochemical hydrogen compressors.

Reduced phase stability and faster formation/dissociation kinetics in confined methane hydrate

The mechanisms involved in the formation/dissociation of methane hydrate confined at the nanometer scale are unraveled using advanced molecular modeling techniques combined with a mesoscale thermodynamic approach. Using atom-scale simulations probing coexistence upon confinement and free energy calculations, phase stability of confined methane hydrate is shown to be restricted to a narrower temperature and pressure domain than its bulk counterpart. The melting point depression at a given pressure, which is consistent with available experimental data, is shown to be quantitatively described using the Gibbs-Thomson formalism if used with accurate estimates for the pore/liquid and pore/hydrate interfacial tensions. The metastability barrier upon hydrate formation and dissociation is found to decrease upon confinement, therefore providing a molecular-scale picture for the faster kinetics observed in experiments on confined gas hydrates. By considering different formation mechanisms-bulk homogeneous nucleation, external surface nucleation, and confined nucleation within the porosity-we identify a cross-over in the nucleation process; the critical nucleus formed in the pore corresponds either to a hemispherical cap or to a bridge nucleus depending on temperature, contact angle, and pore size. Using the classical nucleation theory, for both mechanisms, the typical induction time is shown to scale with the pore volume to surface ratio and hence the pore size. These findings for the critical nucleus and nucleation rate associated with such complex transitions provide a means to rationalize and predict methane hydrate formation in any porous media from simple thermodynamic data.

A new one-site coarse-grained model for water: Bottom-up many-body projected water (BUMPer). II. Temperature transferability and structural properties at low temperature

A number of studies have constructed coarse-grained (CG) models of water to understand its anomalous properties. Most of these properties emerge at low temperatures, and an accurate CG model needs to be applicable to these low-temperature ranges. However, direct use of CG models parameterized from other temperatures, e.g., room temperature, encounters a problem known as transferability, as the CG potential essentially follows the form of the many-body CG free energy function. Therefore, temperature-dependent changes to CG interactions must be accounted for. The collective behavior of water at low temperature is generally a many-body process, which often motivates the use of expensive many-body terms in the CG interactions. To surmount the aforementioned problems, we apply the Bottom-Up Many-Body Projected Water (BUMPer) CG model constructed from Paper I to study the low-temperature behavior of water. We report for the first time that the embedded three-body interaction enables BUMPer, despite its pairwise form, to capture the growth of ice at the ice/water interface with corroborating many-body correlations during the crystal growth. Furthermore, we propose temperature transferable BUMPer models that are indirectly constructed from the free energy decomposition scheme. Changes in CG interactions and corresponding structures are faithfully recapitulated by this framework. We further extend BUMPer to examine its ability to predict the structure, density, and diffusion anomalies by employing an alternative analysis based on structural correlations and pairwise potential forms to predict such anomalies. The presented analysis highlights the existence of these anomalies in the low-temperature regime and overcomes potential transferability problems.

Quantum-mechanical exploration of the phase diagram of water

The set of known stable phases of water may not be complete, and some of the phase boundaries between them are fuzzy. Starting from liquid water and a comprehensive set of 50 ice structures, we compute the phase diagram at three hybrid density-functional-theory levels of approximation, accounting for thermal and nuclear fluctuations as well as proton disorder. Such calculations are only made tractable because we combine machine-learning methods and advanced free-energy techniques. The computed phase diagram is in qualitative agreement with experiment, particularly at pressures ≲ 8000 bar, and the discrepancy in chemical potential is comparable with the subtle uncertainties introduced by proton disorder and the spread between the three hybrid functionals. None of the hypothetical ice phases considered is thermodynamically stable in our calculations, suggesting the completeness of the experimental water phase diagram in the region considered. Our work demonstrates the feasibility of predicting the phase diagram of a polymorphic system from first principles and provides a thermodynamic way of testing the limits of quantum-mechanical calculations.

How ice grows from premelting films and water droplets

Close to the triple point, the surface of ice is covered by a thin liquid layer (so-called quasi-liquid layer) which crucially impacts growth and melting rates. Experimental probes cannot observe the growth processes below this layer, and classical models of growth by vapor deposition do not account for the formation of premelting films. Here, we develop a mesoscopic model of liquid-film mediated ice growth, and identify the various resulting growth regimes. At low saturation, freezing proceeds by terrace spreading, but the motion of the buried solid is conveyed through the liquid to the outer liquid-vapor interface. At higher saturations water droplets condense, a large crater forms below, and freezing proceeds undetectably beneath the droplet. Our approach is a general framework that naturally models freezing close to three phase coexistence and provides a first principle theory of ice growth and melting which may prove useful in the geosciences.

Proton strings and rings in atypical nucleation of ferroelectricity in ice

Ordinary ice has a proton-disordered phase which is kinetically metastable, unable to reach, spontaneously, the ferroelectric (FE) ground state at low temperature where a residual Pauling entropy persists. Upon light doping with KOH at low temperature, the transition to FE ice takes place, but its microscopic mechanism still needs clarification. We introduce a lattice model based on dipolar interactions plus a competing, frustrating term that enforces the ice rule (IR). In the absence of IR-breaking defects, standard Monte Carlo (MC) simulation leaves this ice model stuck in a state of disordered proton ring configurations with the correct Pauling entropy. A replica exchange accelerated MC sampling strategy succeeds, without open path moves, interfaces, or off-lattice configurations, in equilibrating this defect-free ice, reaching its low-temperature FE order through a well-defined first-order phase transition. When proton vacancies mimicking the KOH impurities are planted into the IR-conserving lattice, they enable standard MC simulation to work, revealing the kinetics of evolution of ice from proton disorder to partial FE order below the transition temperature. Replacing ordinary nucleation, each impurity opens up a proton ring generating a linear string, an actual FE hydrogen bond wire that expands with time. Reminiscent of those described for spin ice, these impurity-induced strings are proposed to exist in doped water ice too, where IRs are even stronger. The emerging mechanism yields a dependence of the long-time FE order fraction upon dopant concentration, and upon quenching temperature, that compares favorably with that known in real-life KOH doped ice.

Adsorption of CO and N2 molecules at the surface of solid water. A grand canonical Monte Carlo study

The adsorption of carbon monoxide and nitrogen molecules at the surface of four forms of solid water is investigated by means of grand canonical Monte Carlo simulations. The trapping ability of crystalline Ih and low-density amorphous ices, along with clathrate hydrates of structures I and II, is compared at temperatures relevant for astrophysics. It is shown that when considering a gas phase that contains mixtures of carbon monoxide and nitrogen, the trapping of carbon monoxide is favored with respect to nitrogen at the surface of all solids, irrespective of the temperature. The results of the calculations also indicate that some amounts of molecules can be incorporated in the bulk of the water structures, and the molecular selectivity of the incorporation process is investigated. Again, it is shown that incorporation of carbon monoxide is favored with respect to nitrogen in most of the situations considered here. In addition, the conclusions of the present simulations emphasize the importance of the strength of the interactions between the guest molecules and the water network. They indicate that the accuracy of the corresponding interaction potentials is a key point, especially for simulating clathrate selectivity. This highlights the necessity of having interaction potential models that are transferable to different water environments.

Surface phase transitions and crystal habits of ice in the atmosphere

With climate modeling predicting a raise of at least 2°C by year 2100, the fate of ice has become a serious concern, but we still do not understand how ice grows (or melts). In the atmosphere, crystal growth rates of basal and prism facets exhibit an enigmatic temperature dependence and crossover up to three times in a range between 0° and -40°. Here, we use large-scale computer simulations to characterize the ice surface and identify a sequence of previously unidentified phase transitions on the main facets of ice crystallites. Unexpectedly, we find that as temperature is increased, the crystal surface transforms from a disordered phase with proliferation of steps to a smooth phase with small step density. This causes the anomalous increase of step free energies and provides the long sought explanation for the enigmatic crossover of snow crystal growth rates found in the atmosphere.

Prediction of properties from first principles with quantitative accuracy: six representative ice phases

Based on a high-level MP2 theory with the fragment approach, the crystal structure, vibration spectra and phase transitions of six representative ice phases (II, VI, VII, VIII, IX, and XV) are predicted.

Crystal Structure Prediction via Basin-Hopping Global Optimization Employing Tiny Periodic Simulation Cells, with Application to Water-Ice

A crystal structure prediction algorithm for use in periodic boundary conditions with empirical rigid models is presented, which employs (i) unrestricted cutoff radii for the real-space interactions, thus allowing the treatment of even very small unit cells, and (ii) a global-optimization algorithm based on the basin-hopping method of Wales et al. (D. J. Wales and J. P. K. Doye, J. Phys. Chem. A 1997, 101, 5111). The algorithm is then applied to the TIP4P model of water (W. L. Jorgensen et al., J. Chem. Phys. 1983, 79, 926.) in order to find the lowest enthalpy water-ice crystalline structures in the pressure region 0-8000 bar, in unit cells holding in the range of 1-16 molecules, and a database of the 10 lowest enthalpy structures found at pressures 0, 4000, and 8000 bar is presented. The algorithm finds many of the ice polymorphs and, in particular, finds that the lowest energy structure at zero pressure is almost exactly tied between an ice Ic (cubic ice) and ice Ih (hexagonal ice) structure, having near-identical energies.

Rapid Water Transport through Organic Layers on Ice

Processes involving atmospheric aerosol and cloud particles are affected by condensation of organic compounds that are omnipresent in the atmosphere. On ice particles, organic compounds with hydrophilic functional groups form hydrogen bonds with the ice and orient their hydrophobic groups away from the surface. The organic layer has been expected to constitute a barrier to gas uptake, but recent experimental studies suggest that the accommodation of water molecules on ice is only weakly affected by condensed short-chain alcohol layers. Here, we employ molecular dynamics simulations to study the water interactions with n-butanol covered ice at 200 K and show that the small effect of the condensed layer is due to efficient diffusion of water molecules along the surface plane while seeking appropriate sites to penetrate, followed by penetration driven by the combined attractive forces from butanol OH groups and water molecules within the ice. The water molecules that penetrate through the n-butanol layer become strongly bonded by approximately three hydrogen bonds at the butanol-ice interface. The obtained accommodation coefficient (0.81 ± 0.03) is in excellent agreement with results from previous environmental molecular beam experiments, leading to a picture where an adsorbed n-butanol layer does not alter the apparent accommodation coefficient but dramatically changes the detailed molecular dynamics and kinetics.

Molecular Simulation of the Phase Diagram of Methane Hydrate: Free Energy Calculations, Direct Coexistence Method, and Hyperparallel Tempering

Different molecular simulation strategies are used to assess the stability of methane hydrate under various temperature and pressure conditions. First, using two water molecular models, free energy calculations consisting of the Einstein molecule approach in combination with semigrand Monte Carlo simulations are used to determine the pressure-temperature phase diagram of methane hydrate. With these calculations, we also estimate the chemical potentials of water and methane and methane occupancy at coexistence. Second, we also consider two other advanced molecular simulation techniques that allow probing the phase diagram of methane hydrate: the direct coexistence method in the Grand Canonical ensemble and the hyperparallel tempering Monte Carlo method. These two direct techniques are found to provide stability conditions that are consistent with the pressure-temperature phase diagram obtained using rigorous free energy calculations. The phase diagram obtained in this work, which is found to be consistent with previous simulation studies, is close to its experimental counterpart provided the TIP4P/Ice model is used to describe the water molecule.

GenIce: Hydrogen-Disordered Ice Generator

GenIce is an efficient and user‐friendly tool to generate hydrogen‐disordered ice structures. It makes ice and clathrate hydrate structures in various file formats. More than 100 kinds of structures are preset. Users can install their own crystal structures, guest molecules, and file formats as plugins. The algorithm certifies that the generated structures are completely randomized hydrogen‐disordered networks obeying the ice rule with zero net polarization. © 2017 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.

Computational study of the interplay between intermolecular interactions and CO2 orientations in type I hydrates

Carbon dioxide (CO₂) molecules show a rich orientation landscape when they are enclathrated in type I hydrates. Previous studies have described experimentally their preferential orientations, and some theoretical works have explained, but only partially, these experimental results. In the present paper, we use classical molecular dynamics and electronic density functional theory to advance in the theoretical description of CO₂ orientations within type I hydrates. Our results are fully compatible with those previously reported, both theoretical and experimental, the geometric shape of the cavities in hydrate being, and therefore, the steric constraints, responsible for some (but not all) preferential angles. In addition, our calculations also show that guest-guest interactions in neighbouring cages are a key factor to explain the remaining experimental angles. Besides the implication concerning equation of state hydrate modeling approximations, the conclusion is that these guest-guest interactions should not be neglected, contrary to the usual practice.

Spontaneous NaCl-doped ice at seawater conditions: focus on the mechanisms of ion inclusion

The Cl⁻ ion included in the lattice always substitutes not one but two water molecules favoring ice conduction.

Experimental and theoretical evidence for bilayer-by-bilayer surface melting of crystalline ice

On the surface of water ice, a quasi-liquid layer (QLL) has been extensively reported at temperatures below its bulk melting point at 273 K. Approaching the bulk melting temperature from below, the thickness of the QLL is known to increase. To elucidate the precise temperature variation of the QLL, and its nature, we investigate the surface melting of hexagonal ice by combining noncontact, surface-specific vibrational sum frequency generation (SFG) spectroscopy and spectra calculated from molecular dynamics simulations. Using SFG, we probe the outermost water layers of distinct single crystalline ice faces at different temperatures. For the basal face, a stepwise, sudden weakening of the hydrogen-bonded structure of the outermost water layers occurs at 257 K. The spectral calculations from the molecular dynamics simulations reproduce the experimental findings; this allows us to interpret our experimental findings in terms of a stepwise change from one to two molten bilayers at the transition temperature.

Quest for Inexpensive Hydrogen Isotopic Fractionation: Do We Need 2D Quantum Confining in Porous Materials or Are Rough Surfaces Enough? The Case of Ammonia Nanoclusters

We study the adsorption energetics and quantum properties of the molecular hydrogen isotopes H₂, D₂, and T₂ onto the surface of rigid ammonia nanoclusters with quantum simulations and accurate model potential energy surfaces (PES). A highly efficient diffusion Monte Carlo (DMC) algorithm for rigid rotors allowed us to accurately define zero-point adsorption energies for the three isotopes, as well as the degree of translational and rotational delocalization that each affords on the surface. From the data emerges that the quantum adsorption energy (E_ads) of T₂ can be up to twice the one of H₂ at 0 K, suggesting the possibility of exploiting some form of solid ammonia to selectivity separate hydrogen isotopes at low temperatures (≃20 K). This is discussed by focusing on the structural motif that may be more effective for the task. The analysis of the contributions to E_ads, however, surprisingly indicates that the average kinetic energy (E^kin) and rotation energy (E_rot^kin) of T₂ can also be, respectively, 2 times and 20 times higher than those of H₂; this finding markedly deviates from what is predicted for hydrogen molecules inside carbon nanotubes (CNT) or metallic-organic frameworks (MOF), where E^kin and E_rot^kin is higher for H₂ due to the unavoidable effects of confinement and hindrance to its rotational motion. The rationale for these differences is provided by the geometrical distributions for the rigid rotors, which reveal an increasingly stronger coupling between rotational and translational degrees of freedom upon increasing the isotopic mass. This effect has never been observed before on adsorbing surfaces (e.g., graphite) and is induced by a strongly anisotropic and anharmonic bowl-like potential experienced by the rotors.

Premelting-Induced Smoothening of the Ice-Vapor Interface

We perform computer simulations of the quasiliquid layer of ice formed at the ice-vapor interface close to the ice Ih-liquid-vapor triple point of water. Our study shows that the two distinct surfaces bounding the film behave at small wavelengths as atomically rough and independent ice-water and water-vapor interfaces. For long wavelengths, however, the two surfaces couple, large scale parallel fluctuations are inhibited, and the ice-vapor interface becomes smooth. Our results could help explain the complex morphology of ice crystallites.