Water SPC/E model

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...

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.

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. 

We applied the MD technique to the simulation of brine rejection from freezing salt solutions employing the rigid 3-site simple point charge (SPC/E) water model to obtain the microscopic picture of this very important natural process. Further, we used a recently... 

We performed series of simulations for a range of temperatures around the melting point of the SPC/E water model (215 K, i.e., significantly below the experimental value). Despite this deficiency, this very simple model provides very reasonable description of other water and ice properties. Below, we introduce a relative temperature scale with the melting point assigned a value of 0°. 

Figure 2 The oxygen-oxygen radial distribution functions for bulk water solid line derived from neutron scattering studies (Ref. 28) dashed line obtained from a Monte Carlo simulation employing the SPC/E water model (Ref. 115).

Figure 5.13. Schematic representation of the three-site SPC/E water model. Indicated are the principal axes of the body-fixed reference frame (the x axis being normal to the molecular plane) and the coordinates of constituent atoms. The values of the parameters are zo = —0.0646 A, yu = 0.8165 A, zn = 0.5127 A, 9 = 54.74 °.

Application of the criteria (l)-(4) in molecular dynamic calculations employing the SPC/E water model for obtaining the numbers of HBs per water molecule x at 300 K yielded between 3.2 and 3.4 such bonds. Double-donor water molecules provided according to criterion (3) 1.66 HBs, the rest coming from acceptor molecules (Kumar et al. 2007). These authors also considered electronic structure definitions for the occurrence of HBs between water molecules. Considering the electron donor-acceptor nature of the HB (rather than the hydrogen atom donor-acceptor mode discussed above), the relevant molecular orbital is the o-qh antibonding one of the acceptor that is empty in monomeric water. The occupancy of this orbital, once HBs are formed in liquid water, is the criterion to be used ... 

Balbuena et al. (1998) studied the reorientation times of water molecules over a range of temperatures including supercritical ones, employing semi-continuum molecular dynamics by means of the SPC/E water model (500 water molecules per ion). The reorientation times in bulk water relative to those in the first hydration shell, assuming a coordination number of Nco = 6, are Tiw/Tri = 0.20,0.47,0.65, and 0.90 for Na+, K+, Rb" ", and Cr, respectively, at 25 °C, showing faster reorientation as... 

This potential leads to an improved prediction of the thermodynamics over a wide density and temperature range, and, as seen in Figure 5, the density maximum at 4 °C. (The popular SPC/E water model exhibits a density maximum at —26 The model also produces a good value for the dielectric constant at 81.5 1.5 under ambient conditions. 

The SPC/E water model differs from the SPC only by reparametrization of the site electrostatic charges to account for the missing polarization correction [47]. The vapor-liquid phase envelope was determined through an indirect molecular dynamics approach [67] resulting in a better agreement with the phase diagram of real water in that the critical point of the SPC/E model is -65 IK and p 0.326g / cm. ... 

Figure 3. Comparison of the simulated orientational O - H radial distribution functions for the SPC and SPC/E water models, each shifted up by...

Fig. 6 Potential of mean force between a sodium ion and a chloride ion at infinite dilution in water at 298 K obtained from atomistic simulations with the SPC/E water model (red curve). (Adapted from [75]). The dashed curve shows the Coulomb potential - e lAneocr with a = 71 the static dielectric constant of SPC/E water...

The additional computational effort, compared to fixed partial charges, is rather minor, i.e., about 15% for simulations of neat water [10,11] (the authors consider the TIP4P and SPC/E water models). Figure 3 shows an application of this technique to oligo(ethyleneoxide) in water [12]. This simulation contained a 7mer immersed in 1000 water molecules at ambient... 

R. D. Mountain and A. Wallqvist, A Collection of Results for the SPC/E Water Model, NISTIR 5778, National Institute of Standards and Technology, 1996. (Available from NTIS, Springfield, VA, Order no. PB96-147889/AS.)... 

Fig. 8.12 Radial distribution functions g(r) of the QM water molecule relative to all other MM solvent ones in pure water by (a) the SD QM/MM-MD method and (b) the NAM ones at the HF, BLYP and B3LYP methods with the 6-31+G(d,p) basis set. As a reference, g(r) by the full MM-MD method with the SPC/E water model is also shown (Reproduced with permission from Takenaka et al. [56]. Copyright 2012 Elsevier B.V.)...

Fig. 8.12 the RDF by the full MM-MD method with the SPC/E water model as a reference since it was calibrated to correspond very well to the experimental results [62]. In the RDFs by the SD method (Fig. 8.12a), the first peak is found to exceed the reference (black curve) due to the inaccurate intermolecular interactions between one QM water molecule and close MM water ones in the solute. On the other hand, those by the NAM method with the BLYP and B3LYP methods (Fig. 8.12b) are found to be closer to the reference (black curve), while that with the HF method is lowered due to the neglect of the electronic correlation effect in the QM calculation and the second peaks cannot be accurately reproduced by the NAM method. In this study, we have included four water molecules in the QM region as the first solvation shell, and this discrepancy is probably due to the inaccurate... 

The single point charge (SPC) and extended single point charge (SPC/E) water models have been used with these parameters. Although the bulk properties of SPC/E are better than those of SPC, it has been suggested in the literature that SPC is the better choice for interface studies. ... 

Usually, in water, simulations employing the CHARMM force field are performed using SPC/E water model, and the spine of hydration is well reproduced. 

One unexpected finding is that the SPC and SPC/E water models predict a counter-intuitive density dependence for the oh( ") along the isotherm T, 1.04 in the density range... 

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