ST2 water model

Figure 1 Isotherms of the ST2 water model (shifted by +3.2khar) Solid line shows the experimental transformation LDA —> HDA —> VHDA under isothermal...

The results of early molecular dynamics simulations depend on the model potential functions and the number of particles used for the computations. Geiger (1981) employed the ST2-water model and 215 water molecules/charged particle. 

We thank S. V. Buldyrev and Peter H. Poole for providing original data from computer simulations using the Jagla and ST2 water models, respectively, and for fruitful discussions. Financial support from CUNY, PSC-CUNY-40 award, is acknowledged. 

Figure 6 Fraction of water molecules with NN nearest neighbors in the first coordination shell along the liquid-vapor coexistence curve of ST2 water model [10].

The switching function, Sy (r) has been introduced to reduce unrealistic Coulomb forces between very close water molecules (12). e is the elementary charge and q = 0.23 e is the charge in the ST2 water model, d and r denote distances between point charges and LJ centres, respectively. The choice of a and (3, odd for positive and even for negative charge yields the correct sign. 

Results in Table I illustrate some of the strengths and weaknesses of the ST2, MCY and CF models. All models, except the MCY model, accurately predict the internal energy, -U. Constant volume heat capacity, Cv, is accurately predicted by each model for which data is available. The ST2 and MCY models overpredict the dipole moment, u, while the CF model prediction is identical with the value for bulk water. The ratio PV/NkT at a liquid density of unity is tremendously in error for the MCY model, while both the ST2 and CF models predictions are reasonable. This large error using the MCY model suggests that it will not, in general, simulate thermodynamic properties of water accurately (29). Values of the self-diffusion coefficient, D, for each of the water models except the CF model agree fairly well with the value for bulk water. 

Table 4 3 A comparison various water models [Jorgensen et al 1983], Tor the ST2 potential, q(M) is the charge on the lane pairs , which are a distance 0 8 A from the oxygen atom (see Figure 4 40)...

In molecular dynamics simulations, it is useful to include specific parameters describing the influence of intra- and intermolecular interactions of water molecules. These parameter sets represent water models such as SPC, SPC/E, ST2, TIP3P and TIP4P. Such models were successfully used for the prediction of thermodynamic data of liquid water. If a special water model is chosen in combination with the AMBER force field for calculation of the potential energy according to equation (23.1), term (6) is omitted. 

Figure 5. Maxima and minima lines in the supercritical region of the (a) ST2 water and (b) Jagla models.

Finally, the results reported in Fig. 12 have been used to obtain the H2O density and, by using optical methods, to explore the possibility of a minimum in this quantity. Very recently, using neutron scattering, the existence of a density minimum in the supercooled phase has been observed in confined D2O at Tmm = 210 5K [91]. The idea of a minimum located approximately 70K below density maximum temperature Tma has also been suggested by MD simulation studies ]39] in which both the TIP5P-E and the ST2 potential models for water have been used [39,85]. This possibility can also be inferred from simple arguments. After the maximum, the density of bulk water decreases rapidly with decreasing T before T, whereas ice Ih has a smaller density than that of liquid water and, contrary to the behavior of supercooled bulk water, has a normal positive expansivity, that is, p increases... 

The thermodynamic behavior of both WAC and BKS silica have been examined in the region of the compressibility miiximum, and in both cases, the pattern of thermodynamic anomalies that occur have been found to be the same, and analogous to those found in simulation studies of water. An example is shown in Figure 2, that compares the equation of state features of ST2 water [7] with WAG silica [9] in the P-T plane. In both cases, a retracing line of density maxima occurs above a monotonic spinodal boundary. The qualitative similarity of these features, despite the widely different T and P scales, is striking, and certainly suggests that a search for an LLPT in these silica models, of the kind found in water simulations, is justified. This is discussed in the next section. 

Figure 1 Experimental liquid-vapor coexistence curve of water [3] (thick solid line). Liquid-vapor coexistence curves of several water models ST2 [6,10],ST2RF[6],TIP4P[6],SPCE [6],andSPCE [11]. Fit of the data for the TIP 4P model to the extended scaling equation with leading asymptotic behavior described by eq. (1) is shown by thin solid line.

The temperature of the wetting transition is sensitive to the used water model and to the pore geometry. The phase diagram of ST2 water in slit-like pore of 24 A width and with Uq = -4.62 kcal/mol is shown in Fig. 25. The temperature of the wetting transition is by about 40° higher than in the case of TIP4P water in cylindrical pore with the same water-surface interaction. In parallel, the critical temperature of the prewetting... 

TIP4P, TIP5P SPC models, SPC and SPC/E rigid Stillinger and Rahman model, ST2 and the flexible three-centered water model, F3C. " ... 

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. 

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