SPC/E model

The SPC/E model approximates many-body effects m liquid water and corresponds to a molecular dipole moment of 2.35 Debye (D) compared to the actual dipole moment of 1.85 D for an isolated water molecule. The model reproduces the diflfiision coefficient and themiodynamics properties at ambient temperatures to within a few per cent, and the critical parameters (see below) are predicted to within 15%. The same model potential has been extended to include the interactions between ions and water by fitting the parameters to the hydration energies of small ion-water clusters. The parameters for the ion-water and water-water interactions in the SPC/E model are given in table A2.3.2. 

Table A2.3.2 Halide-water, alkali metal cation-water and water-water potential parameters (SPC/E model). In the SPC/E model for water, the charges on H are at 1.000 A from the Lennard-Jones centre at O. The negative charge is at the O site and the HOH angle is 109.47°.

One of the most convincing tests of the AG relationship appeared in the work of Scala et al.92 for the SPC/E model of water,57 which is known to reproduce many of water s distinctive properties in its super-cooled liquid state qualitatively. In this study, the dynamical quantity used to correlate with the configurational entropy was the self-diffusivity D. Scala et al. computed D via molecular dynamics simulations. The authors calculated the various contributions to the liquid entropy using the methods described above for a wide range of temperature and density [shown in Figure 12(a-c)]. 

Herein we present calculations [6] for liquid H20 that are similar in spirit but different in detail from those of Buch [71, 110] and Torii [97]. The MD simulations are of the SPC/E model [135]. Local-mode anharmonic frequencies are generated from our most recent map developed for the H0D/D20 system [98], as are our transition dipoles. The relatively small intramolecular coupling fluctuates with molecular environment, and is determined by a separate map parameterized from ab initio calculations on clusters. The form of the intermolecular couplings is transition dipole, which is tested and parameterized from additional ab initio calculations. The effects of motional narrowing are taken into account approximately with the TAA [99]. 

T. Bryk, A.D.J. Haymet, Ice lh/water interface of the SPC/E model Molecular dynamics simulations of the equilibrium basal and prism interfaces. J. Chem. Phys. 117, 10258-10268 (2002)... 

Largely motivated by these experiments, we carried out MD simulations using the SPC/E model for water for several thermodynamic states above the critical temperature of model (Tc = 640 K) with densities ranging from 0.05 up to 1.0 g/cm3, as indicated in Table 16-1 [26],... 

The water adsorbed on the surface is described by the SPC model [16]. This fast computable model is well suited for very large systems, as it reproduces quite well the thermodynamical properties around ambient temperature, like vapor pressure (0.044 bar against 0.035 bar experimentaly) and enthalpy of vaporization [17]. The extended SPC/E model [18] is not adapted to study adsorption properties since the polarization correction that it introduces cannot be well defined in the highly inhomogeneous environment of a molecule adsorbed on a surface. Furthermore, the predicted vapor pressure is only half the experimental value [19]. 

Recently, the study of aqueous ionic solutions has been extended to supercritical conditions. Balbuena et al [202] have computed the hydration free energy of several ions (C1 , OH, Na" ", K, Rb" ", Ca " ", Sr " ") using the SPC/E model for water and different ion-water potentials, e.g. OPLS [197] for Cl and Aqvist [190] potentials for cations. They found that hydration free energy of CT is much more affected by the transition from ambient to supercritical conditions than that of Na", due to its stronger electrostatic interaction with water. Also, Balbuena et al observe an overestimated local density for bivalent cations with respect to experimental data [203] that is attributed to differences in concentration and to the potentials adopted. Aqvist potentials, on the other hand, as well as the SPC/E model, have been parametrized to reproduce thermodynamic properties of the ionic solution under ambient conditions and may lack the transferability necessary to describe correctly a solution at supercritical conditions. 

Finally, /o and Cio corresponding to the 12-6 and m-n LJ parameters were found by fitting to eqn. (2). The Lorentz-Berthelot rules were adopted for the description of the cross-interactions, where epo and Coo have the values corresponding to the SPC/E model, 0.1554 kcal/mol and 3.1655A respectively ... 

We simulated the SPC/E basal ice/water interface with Na+ and Cl- ions at a temperature of 225 K, which was estimated as the melting point for the SPC/E model of water in our previous study of two-phase coexistence [19]. We have used a collection of 2304 rigid water molecules plus a single solute ion in the NVT ensemble. The time step was chosen to be 1.5 fs. 

The density-temperature phase diagram for SPC/E water at the pressure of 1 bar has been reported by us earlier [40, 41], The famous density maximum of water, located experimentally at 277 K, is found for this model at the temperature 240 K. Our simulations of two-phase ice/water coexistence for SPC/E model resulted in the melting temperature, which is approximately 50 K below the estimated value of 279 K from the free energy study of melting point for SPC/E water by Arbuckle and Clancy [42], A similar tendency was pointed out by Morris [43], when the melting point from two-phase coexistence simu-... 

Ice surfaces at temperatures close to melting are often said to have a liquidlike layer at the contact with a vacuum [14]. (Fig. 4 above shows that for the SPC/E model at least, this is an over-simplification thee are several different non-ice-like layers at the vacuum.) Therefore it is interesting to study how the Na+ and Cl- ions behave at the ice/vacuum interface. The ions were initially placed at z = 15 A in the MD box, i. e., approximately at the distance of 2 A from the surface. We then allowed the ions to approach the interface over a period of 20 ps, and only after that started the production runs. In order to characterize the location of solute ions with respect to the surface, we introduced a single-ion density profile accumulated over a certain time window. We have found that such single-ion density profiles for cases such as our system, being accumulated over the time of 60 ps, have the width of ss 2 A... 

The extended simple point charge (SPC/E) model [59] is used. This model is known to give reasonably accurate values of static dielectric permittivity of liquid water at ambient conditions [60]. The MD simulations were performed for both H2O and D2O with the system size of 1024 particles at 220 K, 240 K, 267 K, 273 K, 300 K, and 355 K. The parallel molecular dynamics code for arbitrary molecular mixtures (DynaMix) is implemented by Lyubartsev and Laaksonen [61]. The simulations have been carried out on a Linux cluster built on the Tyan/Opteron 64 platform, which enables calculations of relatively long trajectories for a system of 1024 water molecules. The simulation run lengths depend on temperature and are in the range between 1 ns and 4 ns for the warmest and coldest simulation, respectively. As the initial condition was a cubic lattice, the equilibration time was chosen to be temperature dependent in the range from 200 ps at 355 Ktol ns at 200K. 

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