SPC/E Water

Fig. 9.4. Pa (e) and (e) as a function of the binding energy. The simulations treated 216 water molecules, utilizing the SPC/E water model, and the Lennard-Jones parameters for methane were from [63]. The number density for both the systems is fixed at 0.03333 A 3, and T = 298 K established by velocity rescaling. These calculations used the NAMD program (www.ks.uiuc.edu/namd). After equilibration, the production run comprised 200 ps in the case of the pure water simulation and 500 ps in the case of the methane-water system. Configurations were saved every 0.5 ps for analysis...

M. R. Reddy and M. Berkowitz, The dielectric constant of SPC/E water, Chem Phys. 

To compute relative solvation free energies, the solute was first solvated with SPC/E water using the AMBER box option and all solvent molecules located more than 10 A from any of the solute atoms were removed. Water... 

Figure 5 Relationship among loci of structural, dynamic, and thermodynamic anomalies in SPC/E water. The structurally anomalous region is bounded by the loci of q maxima (upward-pointing triangles) and t minima (downward-pointing triangles). Inside of this region, water becomes more disordered when compressed. The loci of diffusivity minima (circles) and maxima (diamonds) define the region of dynamic anomalies, where self-diffusivity increases with density. Inside of the thermodynamically anomalous region (squares), the density increases when water is heated at constant pressure. Reprinted with permission from Ref. 29.

Figure 13 Self-diffusivity D versus configurational entropy, Sc = Scon, for the SPC/E water model at various density p values. The lines are fits to the AG form given by Eq. [10] with tr tx 1/D. Reprinted with permission from Ref. 92.

The behavior of self-diffusivity for these state points is displayed in Figure 12(d). It is clearly visible that the non-monotonic dependence of D on density p is directly reflected by the configurational contribution to the entropy. In fact, the quantitative relationship between D and Sc predicted by Eq. [10] (i.e., the AG relationship) holds remarkably well for SPC/E water over this wide range of thermodynamic conditions, as is shown in Figure 13. 

Table II shows the average end-to-end distance over 20 ps for mannitol and sorbitol in vacuuo and in solution of an argon-like (L-J) solvent and SPC/E water. The average lengths all indicate sickle shapes, except for mannitol in water which is fully extended. This points to a specific solute-solvent interaction between mannitol and water, not just an unspecific solvent effect that is not present in solvent other than water. The model non-aqueous solvent is very artificial, but it should represent the main features of the class of non-polar, spherically symmetric solvents.

Figure 3.17 Comparison between experiment (dashed curve) and calculations combining the polarizable continuum model for solute electronic structure and continuum dielectric theory of solvation dynamics in water. SRF(t) stands for S(t) in our notation. The calculations are for a cavity based on a space-filling model of Cl53, while the experiments are for C343. The two sets of theoretical results correspond to using water e(o>) from simulation (full curve) of SPC/E water and from a fit to experimental data (dash-dotted curve). (Reprinted from F. Ingrosso, A. Tani andJ. Tomasi, J. Mol. Liq., 1117, 85-92. Copyright (2005), with permission from Elsevier).

Figure 3.18 Transverse (a) and (b) longitudinal dipole density time correlations for SPC/E water at 308 K. Results for several k values, ranging from k= 0.2545 A-1 to /c10 = VTo k are shown. Data are from B. M. Ladanyi and B.-C. Perng, in L. R. Pratt and C. Hummer (eds) Simulation and Theory of Electrostatic Interactions in Solution, AIP Conf. Proc., Melville, NY, 1999, Vol. 492, pp. 250-264.

Table 16-1. State parameters and static dielectric properties for SPC/E water [26]...

For the dispersion contribution, we assume that the solute-solvent interaction, in the outer shell, is of the form C/r and that the distribution of water outside the inner shell is uniform. Thus the dispersion contribution is —4TTpC/(3i ), where for the SPC/E water model, 4ttpC/3 is 87.3kcalmol A . The electrostatic effects were modeled with a dielectric continuum approach (Yoon and Lenhoff, 1990), using a spherical cavity of radius R. The SPC/E (Berendsen et al, 1987) charge set was used for the water molecule in the center of the cavity. 

Water dynamics is slowed down by the electric field of the cation, as revealed by diffusion coefficient reduced by a factor of two, compared with pure SPC/E water [132]. A reduction of D of water in ionic solutions is also observed experimentally, with values, determined with the tracer technique, ranging from 1.22 10-5 cm /s for Li+ to 0.52 and 0.53 10 5 cm /s for Fe3+ and Al3+, respectively [206]. 

MD simulations were performed for one ion and 500 SPC/E water molecules. Periodic boundary conditions were imposed and the long-range interactions were accounted for by Ewald summation. 2,83 Xhe ion was kept in a fixed position and the dynamics of the water molecules was calculated. Similarly, for the simulations of pure water, one water molecule was kept in a fixed position, while the dynamics of the surrounding molecules was examined. Static coordination numbers are defined by the expression ... 

Figure 8 The integrated stress-stress autocorrelation function as described in Eqs. [121] for SPC/E water at 303.15 K as described in Ref. 42. Note the convergence of the integral over time deteriorates owing to insufficient data sampling. The experimental value of the shear viscosity is 7.97 x 10 Pa s, whereas the calculated value from this curve 6.6 0.8 x 10 Pa s.

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