Empirical water potentials

SPC/E Simple point charge, extended empirical water potential)... 

Even though from the conceptual point of view the role of Bjerrum defects is well established", their molecular structure and energetics, remains a subject of debate. Recent atomistic studies based on empirical water potentials have provided important qualitative insight into the structure and dynamics of Bjerrum defects, although they have not yet attempted to make direct contact with experimental conductivity data. Furthermore, the few ab initio studies " involve clusters that are too small to reliably capture the properties of a defect embedded in bulk crystal. 

Water is essential to the very existenee of life, playing its important role in a myriad of physieal, ehemical, and biological processes. Despite having a simple moleeular strueture, it forms one of the most eomplex substances. Specific interactions amongn water molecules in the condensed phase are responsible for its anomalous behavior. Extensive atomistic simulations have been performed to connect the microscopic structure of water to its macroscopic properties. The accuracy of simulation results strongly depends on the quality of the applied intermolecular potentials. To this end, more than 50 empirical water potentials have emerged in the literature, broadly differing in the... 

Water Potentials. The ST2 (23), MCY (24), and CF (2J5) potentials are computationally tractable and accurate models for two-body water-water interaction potentials. The ST2, MCY and CF models have five, four, and three interaction sites and have four, three and three charge centers, respectively. Neither the ST2 nor the MCY potentials allow OH or HH distances to vary, whereas bond lengths are flexible with the CF model. While both the ST2 and CF potentials are empirical models, the MCY potential is derived from ab initio configuration interaction molecular orbital methods (24) using many geometrical arrangements of water dimers. The MCY+CC+DC water-water potential (28) is a recent modification of the MCY potential which allows four body interactions to be evaluated. In comparison to the two-body potentials described above, the MCY+CC+DC potential requires a supercomputer or array processor in order to be computationally feasible. Therefore, the ST2, MCY and CF potentials are generally more economical to use than the MCY+CC+DC potential. 

Beside the empirical or semiempirical models described above, the need for inclusion of many-body effects, polarizability at least, in water-water potentials has also been recognized in the development of more recent ab initio potentials [45,75,103-108]. 

All ion-water potentials adopted in the simulations mentioned above are empirical or semi-empirical. In fact, the parameters of the non Coulomb interactions have been fixed either using formation enthalpy data of small clusters in gas phase [118,121,188] or ab initio calculations for the complex geometry [118,188,189] and the position of the first peak of the ion-oxygen 8(r). 

A comparison [216] of experimental total ion-water G(r), a linear combination of partial g(r) s, with the corresponding results from empirical Mg2+-water [190,217] and Ca2+-water potentials [190,217,218] shows that PCM-based potentials produce somewhat less high peaks with a distance of the maximum in better agreement with the neutron diffraction data [219]. All potentials, however, markedly overestimate experimental peak heights, which on the other hand are less accurately determined than their position. 

At the same time, the first empirical trimer potentials fitted to spectroscopic data were developed by Ernesti and Hutson [29,30]. Einally, recently it was possible to develop a three-body potential for the water [34]. Some of this work will be discussed in more detail later on. 

Although there have been a fairly large number of first-principle simulations of condensed phase published to date, this number is completely dwarfed by simulations based on empirical potentials. The popular empirical pair potentials for water [76-78] have been used in many (thousands) research projects. The empirical potentials are usually fitted in simulations for liquids to reproduce measured properties of this phase. Thus, these potentials mimic the nonadditive effects by distortions of two-body potentials... 

The more recent ASP potentials of MUlot et al. [183] and their counterparts fitted to the experimental dimer spectra, VRT(ASP-W) [52] and VRT(ASP-W)III [53], have been utihzed in diffusion Monte Carlo (DMC) simulation of water clusters of different sizes [192,193]. The three- and higher body effects were described by a polarization model only, similarly as in empirical polarizable potentials. While polarization models are quite efficient in describing the nonadditive induction in the asymptotic regime, they fail to properly model the short-range nonadditivities, which are definitely non-negligible in smaller trimers. 

The first comment refers to the work of Cho et al. (1996), who used an earlier version of the primitive model for water (the one referred to as the BNID potential in Fig. 2.10c). In their conclusion, the authors wrote It might be necessary, in order to reproduce the known temperature and pressure effects, to insert empirically the appropriate double-well feature into the water potential. (italics added)... 

Water-water potential functions have been reported from different laboratories. The procedure leads either to analytic functions fitted to empirical data, e.g. SPC (single point charge) [19] and TIPS (transferable intermolecular potential functions)... 

The reaction path of the enzyme TIM was characterized by energy minimizations using the combined AMl/CHARMM potential.28 >285 in that study, the substrate and the catalytic side chains of His-95 and Glu-165 were treated quantum mechanically, and all other molecules in a stochastic boundary system (a 16-A sphere) were treated by the empirical CHARMM potential. The entire system consisted of 1250 protein atoms and 100 water molecules. An energy decomposition method was used to locate residues important in catalysis. In addition to the large energy contributions from conserved residues, other amino acids make smaller contributions, but their cumulative effect is significant. Interestingly, a number of residues far from the active site also have sizable effects on the enzyme s catalytic ability.284... 

Understanding the hydrogen bond properties of water has been a grand challenge for liquid state theories and molecular simulations. The first simulation of liquid water was carried out more than four decades ago by using the method of Monte Carlo, where the energies of different configurations were calculated from empirical interaction potentials. However, the many-body interactions and their participation in... 

The ab initio molecular dynamics technique provides a powerful method in studying the properties of chemical systems under varying thermodynamic conditions without having to employ any empirical interaction potentials. In this chapter, a brief review has been made on our recent studies on water dynamics by using this method combined with a time series analysis. We have discussed the frequency-structure correlations of water molecules in both supercritical and normal water. Our calculations reveal that hydrogen bonds still persist to some extent in the supercritical state. However, the quantitative details of hydrogen bonding depend on the density. At... 

Dimer properties derived from the different types of potential discussed earlier in the section on parameter fitting differ from each other depending on their type of parameterization. In Table 3 we can see that an empirical liquid water potential tends to overestimate the binding energy, whereas the geometry is more akin to typical dimer configurations in a liquid environment. Grafting a... 

Scheraga and colleagues have continued work on the structure of water. Shipman and Scheraga (1974) referred to the many empirical intermolecular potentials that have been developed. One by Ben-Naim and Stillinger appears to have had some success, although several defects were stated to have been detected. This potential was based on classical mechanics, whereas it now appears that the intermolecular motions among water molecules are of a quantum-mechanical... 

The combined simultaneous growth over the last 10 years in the area of ab initio theory, and the development of efficient electronic structiue software suites that take advantage of parallel hardware architectures, have significantly advanced our abihty to obtain accurate energetics for medivun size (up to 30 molecules) clusters of water molecules. This advance provides a new route in the development of empirical interaction potentials for water, especially in the absence of experimental information regarding the cluster... 

Customarily, it is assumed that e is unity and that ]l = p,cos 9, where 0 is the angle of inclination of the dipoles to the normal. Harkins and Fischer [86] point out the empirical nature of this interpretation and prefer to consider only that AV is proportional to the surface concentration F and that the proportionality constant is some quantity characteristic of the film. This was properly cautious as there are many indications that the surface of water is structured and that the structure is altered by the film (see Ref. 37). Accompanying any such structural rearrangement of the substrate at the surface should be a change in its contribution to the surface potential so that AV should not be assigned too literally to the film molecules. 

---