Nuclear quantum effects influence the structure and dynamics of hydrogen bonded systems, such as water, which impacts their observed properties with widely varying magnitudes. This review highlights the recent significant developments in the experiment, theory and simulation of nuclear quantum effects in water. Novel experimental techniques, such as deep inelastic neutron scattering, now provide a detailed view of the role of nuclear quantum effects in water's 2 properties. These have been combined with theoretical developments such as the introduction of the competing quantum effects principle that allows the subtle interplay of water's quantum effects and their manifestation in experimental observables to be explained. We discuss how this principle has recently been used to explain the apparent dichotomy in water's isotope effects, which can range from very large to almost nonexistent depending on the property and conditions. We then review the latest major developments in simulation algorithms and theory that have enabled the efficient inclusion of nuclear quantum effects in molecular simulations, permitting their combination with on-the-fly evaluation of the potential energy surface using electronic structure theory. Finally, we identify current challenges and future opportunities in the area.3

In this work we show that homogeneous nucleation of methane hydrate can, under appropriate conditions, be a very rapid process, achieved within tens of nanoseconds. In agreement with recent experimental results on different systems, we find that the nucleation of a gas hydrate crystal appears as a two-step process. It starts with the formation of disordered solid-like structures, which will then spontaneously evolve to more recognizable crystalline forms. This previously elusive first-stage state is confirmed to be post-critical in the nucleation process, and is characterized as processing reasonable short-range structure but essentially no long-range order. Its energy, molecular diffusion and local structure reflect a solid-like character, although it does exhibit mobility over longer (tens of ns) timescales. We provide insights into the controversial issue of memory effects in methane hydrates. We show that areas locally richer in methane will nucleate much more readily, and no 'memory' of the crystal is required for fast re-crystallization. We anticipate that much richer polycrystallinity and novel methane hydrate phases could be possible.

Liquid state structure has traditionally been characterized with the radial distribution functions between atoms. Although these functions are routinely available from x-ray diffraction and neutron scattering experiments or from computer simulations, they cannot be interpreted unambiguously to provide the spatial order in a molecular liquid. A direct approach to determining the spatial structure in the liquid state is demonstrated here. Three-dimensional maps representing the local atomic densities are presented for several water models. These spatial maps provide a picture of the short-range order in liquid water which reveals specific details of its local structure that are important in the understanding of its properties.

Despite the fact that an enormous literature has now accumulated on the structure in liquid water, the focus has been primarily limited to the average radial distributions of particles; local (atomic) pair-density maps which span both the radial and the angular coordinates of the separation vector have remained largely unexplored. In this work, we have obtained the spatial distribution functions gOO(r,Ω) and gOH(r,Ω) for liquid water and have applied them to an analysis of the equilibrium structure. Molecular dynamics simulations of SPC/E water have been carried out at temperatures of −10, 25, and 100 °C and the local liquid structure examined. It is found that the unfolded O...O distribution demonstrates, in addition to peaks consistent with a continuous tetrahedral network pattern, a distinct maximum in the local atomic pair density at ‘‘interstitial’’ separations of about 3.5 Å. This local maximum is lost in the spatially folded radial distribution function gOO(r) due to averaging over the entire angular space. By examining the peaks in gOO(r,Ω) due to nearest neighbors, we have shown that the tetrahedral network coordination number in liquid SPC/E water equals 4.0 and does not depend on temperature. The average number of molecules in additional nontetrahedral coordination, which is found to vary with temperature, has also been extracted, enabling us to establish full average coordination numbers of 4.8–5.0 in the temperature range of −10–100 °C. In addition, we have determined and analyzed statistical distributions for the pair energies and H-bond angles for different water fractions as identified from gOO(r,Ω) and gOH(r,Ω).

Articles you may be interested inElectrolyte diodes with weak acids and bases. I. Theory and an approximate analytical solution On the molecular theory of aqueous electrolyte solutions. IV. Effects of solvent polarizability J. Chem. Phys. 92, 1345 (1990); 10.1063/1.458145 On the molecular theory of aqueous electrolyte solutions. II. Structural and thermodynamic properties of different models at infinite dilution J. Chem. Phys. 89, 5843 (1988); 10.1063/1.455535 Simple electrolytes in the mean spherical approximation. III. A workable model for aqueous solutionsThis paper describes a theoretical study of the thermodynamic, dielectric, and structural properties of model aqueous electrolyte solutions. The model considered consists of hard sphere ions immersed in a hard polarizable dipole tetrahedral-quadrupole solvent with waterlike parameters. The calculations involve the solution of the reference hypemetted-chain (RHNC) approximation for ion-solvent mixtures at finite concentration and some details of the general method are discussed. The influence of the molecular polarizabiIity of the solvent particles is treated at the self-consistent mean field (SCMF) level and, surprisingly, the mean dipole moment of the solvent is found to be nearly independent of the salt concentration. Numerical results are reported for model alkali halide solutions and other 1: 1 electrolytes, and comparisons are made with experimental results at 25 ·C. The agreement obtained between theory and experiment is variable depending upon the particular property and solution considered. In addition to the explicit numerical results for aqueous electrolytes several general analytical results are also given. The most interesting of these are expressions for the low concentration large separation limiting behavior of the ion-solvent and solvent-solvent radial distribution functions. caP (12) = haP (12) -lngaP (12) -u ap (12)lkT,

The diverse properties of hydrogen-bonded liquids and solutions must manifest their unique local structures. An unambiguous three-dimensional picture of the local ordering in these liquid systems is not accessible through radial distribution functions, the usual outputs of computer simulation, or experimental studies. In this work we employ spatial distribution functions to analyze the three-dimensional local structure in water−methanol solutions. Molecular dynamics simulations are performed at room temperature for five water−methanol liquid mixtures scanning the entire range of compositions. The effects of the alcohol on water structure and water on methanol structure are considered in detail. The results are compared to previous simulations and discussed from the point of view of various solvation models. Large structural changes are observed, many of which are not apparent from simple radial analysis. In water-rich solution we confirm a high degree of ordering, characterized by a very strong preference for tetrahedral arrangements, where the water molecules appear most highly localized around the hydroxyl group of the methanol solute. Strongly hydrated methanol molecules adopt rather specific relative positions that most readily accommodate the ordering within their hydration cages. In methanol-rich solution the local structure very closely resembles that of pure methanol. We find that rather long equilibration periods appear to be necessary to obtain accurate structural information in computer simulations of these complex systems.

In this Letter we report our success in crystallizing a bulk sample of liquid water in molecular dynamics simulations.In these computer experiments supercooled liquid TIP4P water at 250 K was subjected to a homogeneous static electric field; the resulting polar crystal which forms within 200 ps has the structure of ice I,. These simulation results suggest that the local electric fields that exist near the surfaces of various materials or within confined geometries can play an important role in promoting the crystallization of liquid water. PACS numbers: 61.25.Em, 64.60.Qb, 64.70.Dv Although the structure and physical properties of liquid water at room temperature [1 -3] are now well established, a fundamental understanding of its lowtemperature behavior [4 -6] is still lacking. Upon freezing, water can form various crystalline and amorphous ices [2, 4 -6]. Under certain conditions these solids exhibit a complex series of phase transformations [4 -8] which, together with the thermodynamic anomalies of the supercooled liquid [4,6], remain the subject of intense debate. In recent years computer simulations have contributed greatly to this area. Among many such studies [6,8 -13] the enormous computational efforts undertaken by Poole, Sciortino, Essmann, and Stanley [6,8] to model metastable phases of solid water are of particular note. In their work they have used molecular dynamics (MD) simulations to help construct a phase diagram for low-and high-density amorphous ice. However, despite these tremendous computational efforts, no one had yet succeeded in crystallizing a bulk sample of water in a computer simulation.We are aware of no simulation study in which any isotropic molecular liquid was continuously transformed into a regular solid. It is not surprising then that freezing phenomena [14],although among the most fundamental in nature, still remain rather poorly understood at the microscopic level.Thermodynamics views the transformation of a molecular liquid into a molecular crystal as a first-order phase transition. In the absence of a foreign surface or particle to promote heterogeneous nucleation, this phase transition begins with the spontaneous nucleation of molecules into small aggregates which, if they reach a critical size, form the seeds for the new phase. Unfortunately, computer simulation methods have been unable to model this dynamical process directly, the large-scale density and polarization fluctuations that might initiate formation of a heterophase have not been recorded even for very large systems. Upon reducing the temperature, phase trajectories tend to get trapped into a region of rnetastable glassy states [6] which makes it impossible to follow further the evolution of a molecular system towards its crystalline state.We have attempted to bypass these limitations by applying a static electric field in MD simulations of TIP4P [15] water and have discovered that the deeply supercooled liquid can be completely transformed into a polar crystal with the regular structure of ice I within only a f...

In this paper we report a successful molecular simulation study exploring the heterogeneous crystal growth of sI methane hydrate along its [001] crystallographic face. The molecular modeling of the crystal growth of methane hydrate has proven in the past to be very challenging, and a reasonable framework to overcome the difficulties related to the simulation of such systems is presented. Both the microscopic mechanisms of heterogeneous crystal growth as well as interfacial properties of methane hydrate are probed. In the presence of the appropriate crystal template, a strong tendency for water molecules to organize into cages around methane at the growing interface is observed; the interface also demonstrates a strong affinity for methane molecules. The maximum growth rate measured for a hydrate crystal is about 4 times higher than the value previously determined for ice I in a similar framework (Gulam Razul, M. S.; Hendry, J. G.; Kusalik, P. G. J. Chem. Phys. 2005, 123, 204722).