Water TIP4P model

The simulated free surface of liquid water is relatively stable for several nanoseconds [68-72] because of the strong hydrogen bonds formed by liquid water. The density decrease near the interface is smooth it is possible to describe it by a hyperbolic tangent function [70]. The width of the interface, measured by the distance between the positions where the density equals 90% and 10% of the bulk density, is about 5 A at room temperature [70,71]. The left side of Fig. 3 shows a typical density profile of the free interface for the TIP4P water model [73]. 

Table 17.5 Comparison of interaction energies of the literature C02 models, using the TIP4P water model, to the ab initio calculations (interaction energies given in kcal/mol) ...

Fig. 17.6 A free energy curve generated from varying the O-a values, while holding the well depth constant, in the bulk solution parameterization process using the TIP4P water model...

Computer simulations have over the years contributed to our understanding of the liquid water/vapor interface. A fairly early study by Wilson et al. (1987) used the TIP4P water model with 342 molecules at a rather high temperature, 52 °C. They confirmed the preferred dipole orientation at the surface with the positive end (hydrogen atoms) towards the bulk liquid and considered the interfacial region to extend 0.75 nm into the liquid. However, the calculated surface tension, 132 46 mN m , is nearly twice the experimental value, so that the implementation of the model needed revision. Subsequent work by others improved on this situation. Taylor et al. (1996) used the SPC/E model of water with 526 molecules at several temperatures from —5 to 100 °C (and also with 1,052 molecules at 25 °C) at 0.1 MPa. A main conclusion is... 

Thus, the polarizable models used in the simulation of supercritical water typically involve [78,81,83,85,86] the rigid geometry of either the SPC or the TIP4P water model, i.e., a planar configuration with an HOH angle of either 109.5 ... 

The thermodynamic free energy perturbation route was used by Gao [228] in the analysis of the benzene dimer formation in supercritical water at 673K and 35MPa, via NPT Monte Carlo simulation of TIP4P water model and OPLS benzene. The resulting PMF indicated no local water density increase around benzene, as if benzene behaved as a weakly repulsive solute [187]. In fact, the calculated association constant was approximately three times smaller than that corresponding to water at ambient conditions. 

Despite these fairly accurate dipole moments, DFT molecular electrostatic potential fitted charges are most likely not appropriate for simple additive molecular mechanical force fields. For these types of force fields to be accurate in aqueous environments, a priori polarization of molecules is required. The DFT charges are too gas phase-like, and initial tests indicate that the charges are too small and their electrostatic interactions with prepolarized water molecules (such as the SPC, TIP3P, and TIP4P water models ) are too weak. On a brighter note, as nonadditive, polarizable, force fields become more popular, the ability to accurately reproduce gas phase dipole moments will be extremely desirable. 

IR and Raman line shapes of ice Ih have also been recently studied by Skinner et al. [118-121]. The results of their calculations for isotopically substituted HOD in either H O or D O were presented in Ref. 118. The calculations, which exploit a mixed quantum/classical approach and a new TIP4P water model [122], yield... 

The OPLS term also includes the TIP3P and TIP4P water models of Jorgensen (W. Jorgensen, J. Chandresekhar, J. Madura, R. Impey, and M. Klein, J. Chem. Phys., 1983, 79, 926-935). 

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