TIP4P potential

Kathmann et al. [89,90] claim based on classical MC simulations with TIP4P potential never intended to treat small clusters [64] that the effect of the anhar-monicity on free energies of small clusters is large. This claim contradicts the results of quantum-mechanical DFT and ab initio studies [65,98,108,115]. 

The water-water intermolecular interaction is described by the TIP4P potential. The ethane molecule consists of two interaction sites, each of which interacts with each other via Lennard-Jones (LJ) potential. The reference of ethane molecule is spherical and is of LJ type interaction with size and energy parameters of 4.18 A and 1.72 kj/mol. The LJ parameters for methyl group of ethane are 3.78 A and 0.866 kj/mol. For the water-guest interaction, the Lorentz-Berthelot rule is assumed. The interaction potentials for all pairs of molecules are truncated smoothly at... 

Figure 3 Above Radial distribution functions for water samples at 25 C using the TIP4P potential (solid line) and the TIP4P potential plus a localized repulsive barrier (dashed line), as discussed in the text. Below Constant potential surfaces of the localized repulsion added to the TIP4P model. The surfaces correspond to an energy value of 1 kT.

M. Neumann, Dielectric relaxation in water. Computer simulations with the TIP4P potential, J. Chem. Phys., 85 (1986) 1567-1580. 

Fig. 6. MD simulations for ice Ih with different sizes of super-lattice cells, 64, 128, 256 and 512 water molecules using TIP4P potentials. The calculations show that intensities for 64 and 128 molecules are very noisy . The 512 molecule cell shows a good agreement with LD simulation result see Fig. 16, indicating the BZ integration is about acceptable with at least 512 molecules.

Recently, there were many attempts of MD simulations for the vibrational dynamics of ice. In these calculations more realistic, either non-rigid or polarizable, potentials were used. One such calculation was made by Itoh et al [72] using the KKY potential [9] which has three separate pair-wise terms yoo(r), VoH(r), VnH(r) and an extra three-body term for H-O-H and H-0—H bending. These calculations produced the all the fundamental modes up to 450 meV (or 3622 cm" ). The resulting spectra show very similar features to results from the MCY and TIP4P potentials in the translational and librational regions (see Fig. 16 and 17). 

Figure 3 Potential energies of PWCs versus total dipole moments for complete classes of discrete optimized configurations. Mainly the values are calculated using TIP4P potential.

Figure 1 The left plot represent total dipole moment versus a number of strong H-bonds for minimal unit cell of structure I. The right diagram shows the energy distribution of local optimized configurations (TIP4P potential).

We first considered the water hexamer, which is the smallest cluster for which a noncyclic structure is found to be the global minimum, both theoretically [318-321] and experimentally [322]. For the TIP4P potential we have probably located the majority of the minima and transition states on the PES, while for ASP-W4 there are many more stationary points, and our sampling of the higher energy region of the surface is incomplete. For TIP4P... 

Figure 2. Thermodynamic properties of supercritical water simulated using the TIP4P potential. Symbols are MC results, lines are calculated from the standard equation of state for water (Saul and Wagner 1989).

Figure 7. Atom-atom radial distribution functions for liquid and supercritical water at 1.0 g/cm and 0.17 g/cm from MC simulations with the TIP4P potential.

Figure 8 (left). Atom-atom radial distribution functions for subcritical water (T=573 K, p=0.72 g/cm ) from MC simulations with the TIP4P potential (solid lines). Thick dotted hne - X-ray diffraction (Gorbaty and Demianets 1983) thin dotted lines with error bars - neutron diffraction with isotope substitution, NDIS (Soper et al. 1997). [Reproduced with permission from JPhys Chem A 1997, 101, 9720-9727. 1997 American Chemical Society.]... 

The chemical potential difference is tabulated in Table III [17,18]. There are small but nonmegligible differences depending on both the method and the pair potential. Two distinctive features are noteworthy. CS-II is more stable than CS-I irrelevant as to whether the anharmonic free energy is taken into account for the TIP4P potential (all the properties are calculated for clathrate hydrates with this potential unless otherwise mentioned) [52]. This is also true for most of the other pair potential of water such as SPC/E [53]. The chemical potential difference is negative for most of the potential but it is positive for the CC potential [54] and other potentials with the same functional form, which is favorable to observe a hydration structure around a hydrophobic solute but is inappropriate to evaluate the thermodynamic stability of clathrate hydrates. 

The initial efforts led to development of the TIP3P and TIP4P potential functions for water, which quickly became and remain the most widely used models for water. Monte Carlo results were also reported for more than 40 organic liquids, most for the first time, that provided unprecedented detail on the structures and conformational properties of the... 

MP2/CBS enthalpies were obtained in the harmonic approximation using the MP2/aug-cc-pVTZ harmonic frequencies. We now note a significant difference between the two models, which arises from the inability of the effective TIP4P potential to describe the cooperative effects in small water clusters. 

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