Potential energy surfaces liquid water

Numerous studies on water have been mentioned above. For liquid water, see 67, 73, 108,109 for potential energy surfaces 40,107—109,133,141,148,149,158 160 for simulations and properties and 265, 273, 284, 289 for phase diagrams and transitions. For aqueous solutions, see 83, 86 for potential energy surfaces and 111, 134, 293, 315, 357 for simulations. Water and solutions in pores and membranes is discussed in 314, 356, 441-443, 475. However, due to the huge amount of interest in liquid water and aqueous solutions there is much that has not been covered so far. Work has been carried out on water due to its importance, but also because it is a challenging system from theoretical, simulation and experimental perspectives. Many new potential energy surfaces for water appear in the literature each year, and it is usually one of the first systems on which new simulations are tested. Here we mention some of the important studies that have not been considered above. 

Goldman, N. Leforestier, C. Saykally, R. J., A firstprinciples potential energy surface for liquid water from VRT spectroscopy of water clusters, Philos. Trans. R. Soc. A 2005, 1-16. doi 10.1098/rsta.2004.1504... 

Most of the potential energy surfaces reviewed so far have been based on effective pair potentials. It is assumed that the parameterization is such as to account for nonadditive interactions, but in a nonexplicit way. A simple example is the use of a charge distribution with a dipole moment of 2.ID in the ST2 model. However, it is well known that there are significant non-pairwise additive interactions in liquid water and several attempts have been made to include them explicitly in simulations. Nonadditivity can arise in several ways. We have already discussed induced dipole interactions, which are a consequence of the permanent diple moment and polarizability of the molecules. A second type of nonadditive interaction arises from the deformation of the molecules in a condensed phase. Some contributions from such terms are implicitly included in calculations based on flexible molecule potentials. Other contributions arises from electron correlation, exchange, and similar effects. A good example is the Axilrod-Teller three-body dispersion interaction ... 

Three types of surface are in use for water simulations. The first consists of simple empirical models based on the LJ-C potential. There seems to be no purpose in continuing to develop and use such models as they give little, if any, new information. A second group attempts to improve the accuracy of the potential using semiempirical methods based on a comprehensive set of experimental data. These models allow for physical phenomena such as intramolecular relaxation, electrostatic induced terms, and many-body interactions, all of which are difficult to incorporate correctly in liquid water theories. There is room for much more work in these areas. The third group makes use of the most advanced ab initio methods to develop accurate potentials from first principles. Such calculations are now converging with parameterized surfaces based on accurate semiempirical models. Over the next few years it seems very likely that the continued application of the second and third approaches will result in a potential energy surface that achieves quantitative accuracy for water in the condensed phase. 

Two methods are in common use for simulating molecular liquids the Monte Carlo method (MC) and molecular dynamics calculations (MD). Both depend on the availability of reasonably accurate potential energy surfaces and both are based on statistical classical mechanics, taking no account of quantum effects. In the past 10-15 years quantum Monte Carlo methods (QMC) have been developed that allow intramolecular degrees of freedom to be studied, but because of the computational complexity of this approach results have only been reported for water clusters. 

T. Komatsuzaki and I. Ohmine, Energetics of proton transfer in liquid water. I. Ab initio study for origin of many-body interactions and potential energy surfaces, Chem. Phys., 180 (1994) 239-269. 

Methods for simulation of the liquid-vapour coexistence are well developed and were reviewed by Panagiotopoulos263 however in some cases these have been shown to be sensitive to the potential energy surface and factors such as many-body interactions, and therefore new results continue to be obtained to investigate these issues and to study new systems (see for example,110 for mercury,264 for methane, and265 for water). The Gibbs ensemble approaches and grand canonical and isothermal-isobaric MC simulations with histogram... 

A wide variety of different models of the pure water/solid interface have been investigated by Molecular Dynamics or Monte Carlo statistical mechanical simulations. The most realistic models are constructed on the basis of semiempirical or ab initio quantum chemical calculations and use an atomic representation of the substrate lattice. Nevertheless, the understanding of the structure of the liquid/metal surface is only at its beginning as (i) the underlying potential energy surfaces are not known very well and (ii) detailed experimental information of the interfacial structure of the solvent is not available at the moment (with the notable exception of the controversial study of the water density oscillations near the silver surface by Toney et al. [140, 176]). 

Surface tension results from the tendency of liquids to minimize their surface area in order to maximize the interactions between their constituent particles, thus lowering potential energy. Surface tension causes water droplets to form spheres and allows insects and paper clips to float on the surface of water. 

The study of liquids near solid surfaces using microscopic (atomistic-based) descriptions of liquid molecules is relatively new. Given a potential energy function for the interaction between liquid molecules and between the liquid molecules and the solid surface, the integral equation for the liquid density profile and the liquid molecules orientation can be solved approximately, or the molecular dynamics method can be used to calculate these and many other structural and dynamic properties. In applying these methods to water near a metal surface, care must be taken to include additional features that are unique to this system (see later discussion). 

Another well-known example is the floating of a metal needle (heavier than water) on the surface of water (Figure 1.4.) The surface of a liquid can thus be regarded as the plane of potential energy. It may be assumed that the surface of a liquid behaves as a membrane (at a molecular scale) that stretches across and needs to be broken in order to be penetrated. One observes this tension when considering that a heavy iron needle (heavier than water) can be made to float on the water surface when carefully placed (Figure 1.4). 

Mass Accommodation Coefficient. For a given molecule the mass accommodation coefficient is a physical constant which depends only on the temperature and on the nature of the liquid surface. The process of the molecule entering the liquid phase might proceed as follows. Since the surface of water is non-rigid it is likely that a molecule which strikes the surface achieves thermal accommodation with near-unit probability. The molecule is bound to the surface in a potential well of depth aU, where aUs is the binding energy of the molecule to the liquid surface. 

---