Nonpolarizable Models

Before discussing polarizable models, a useful starting point is to consider nonpolarizable models. A typical nonpolarizable potential for molecular systems is  

Another way in which polarizability is included implicitly is in the value of the partial charges, qi, that are assigned to the atoms in the model. The charges used in potential energy models for condensed phases are often enhanced from the values that would be consistent with the gas-phase dipole moment, or those that would best reproduce the electrostatic potential (ESP) 

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The magnitude of such electrostatic effects makes the question of ionic polarizability particularly important. Simulations have shown that while simulations employing nonpolarizable models adequately reproduce equilibrium properties in fused salts [86], ionic polarizability may have important... 

Most nonpolarizable water models are actually fragile in this regard they are not transferable to temperatures or densities far from where they were parameterized. Because of the emphasis on transferability, polarizable models are typically held to a higher standard and are expected to reproduce monomer and dimer properties for which nonpolarizable liquid-state models are known to fail. Consequently, several of the early attempts at polarizable models were in fact less successful at ambient conditions than the benchmark nonpolarizable models, (simple point charge) and TIP4P (transfer-... 

It is now well understood that the static dielectric constant of liquid water is highly correlated with the mean dipole moment in the liquid, and that a dipole moment near 2.6 D is necessary to reproduce water s dielectric constant of s = 78 T5,i85,i96 holds for both polarizable and nonpolarizable models. Polarizable models, however, do a better job of modeling the frequency-dependent dielectric constant than do nonpolarizable models. Certain features of the dielectric spectrum are inaccessible to nonpolarizable models, including a peak that depends on translation-induced polarization response, and an optical dielectric constant that differs from unity. The dipole moment of 2.6 D should be considered as an optimal value for typical (i.e.. 

Dynamic properties, such as the self-diffusion constant, are likewise strongly correlated with the dipole moment. This coupling between the translational motion and the dipole moment is indicated in the dielectric spectrum. Models that are overpolarized tend to undergo dynamics that are significantly slower than the real physical system. The inclusion of polarization can substantially affect the dynamics of a model, although the direction of the effect can vary. When a nonpolarizable model is reparameterized to include polarizability, the new model often exhibits faster dynamics, as with polarizable versions of TIP4P, ° Reimers-Watts-Klein and reduced... 

Liquid Water Using Polarizable and Nonpolarizable Models. 

A. Wallqvist and B.J. Berne, Effective potentials for liquid water using polarizable and nonpolarizable models, J. Phys. Chem., 97 (1993) 13841-13851. 

The simulations with both models demonstrated the existence of an inhomogeneous interfacial structure normal to the surface layer. In the outmost surface layer, the cation is likely to lie on the surface with the imidazolium ring parallel to the interface, while there is a second region with enhanced density from that in the bulk where the cation tended to be perpendicular to the surface. It was found that the cation is likely to be segregated at the IL surface for the polarizable model, while for the nonpolarizable model, the anion was found to be more likely exhibiting such behavior. The surface tension obtained from the polarizable model was much smaller (>28%) than that obtained from the nonpolarizable model, in better agreement with extrapolated experiments [107],... 

In summary, our simulation results provide strong support to the contention that rigid-planar nonpolarizable models of water suffer from an inherent transferability problem due to their inability to adjust their interaction strength to the actual polarizing environment. None of this type of models is capable of predicting correct critical data, vapour pressure or second virial coefficient. None of the models tested so far predicts the difference of the melting and the liquid density maximum temperature accurately. 

Veldhuizen and de Leeuw (1996) used the OPLS parameters for methanol and both a nonpolarizable and a polarizable model for carbon tetrachloride for MD simulations over a wide range of compositions. The polarization contribution was found to be very important for the proper description of mixture properties, such as the heat of mixing. A recent study by Gonzalez et at (1999) of ethanol with MD simulations using the OPLS potential concluded that a nonpolarizable model for ethanol is sufficient to describe most static and dynamic properties of liquid ethanol. They also suggested that polarizabilities be introduced as atomic properties instead of the commonly approach of using a single molecular polarizability. 

K. A. Motakabbir and M. Berkowitz, Chem. Phys. Lett., 176, 61 (1991). Liquid-Vapor Interface of TIP4P Water Comparison Between a Polarizable and a Nonpolarizable Model. 

So far, the multipole approach has been rather limited for modeling liquid water. For rigid, nonpolarizable models, the SSDQOl discussed here uses multipoles up to the octupole that have been optimized for various properties of liquid water at STP [29]. In addition, SCME, a polarizable molecular multipole model using multipoles up to the hexadecapole, has recently been developed [49]. 

Motakabbir KA, Berkowitz ML (1991) Liquid-vapor interface of TIP4P water comparison between a polarizable and a nonpolarizable model. Chem Phys Lett 176 61-66 Mountain RD (1989) Molecular dynamics investigation of expanded water at elevated temperatures. J ChemPhys 90 1866-1870... 

Our simulations show that the interfacial region has a narrow water-hydrocarbon interface about 4 A thick, as has been found in previous simulation of micelles [9, 27]. The Na" " ions and carboxylate head groups make up a characteristic electrostatic double layer around these micelles. In the polarizable models the Na" ions do not come in contact as much with the headgroups as in the original nonpolarizable model, in better agreement with experiment. 

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