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,