Computational polarity modelling

Kaminski GA, Friesner RA, Zhou RH (2003) A computationally inexpensive modification of the point dipole electrostatic polarization model for molecular simulations. J Comput Chem 24(3) 267-276... 

Russell et al. (1992) conducted a similar study using the same data set from Hine and Mookeijee (1975). They developed a computer-assisted model based on five molecular descriptors which was related to the compound s bulk, lipophilicity, and polarity. They found that 63 molecular stmctures were highly correlative with the log of Henry s law constants (r = 0.96). 

There has been a growing interest in the food industry for a color space based on a polar model. In 1976 when CIELAB was adopted, the CIE recommended an alternative color scale known as CIELCH or L C H. Of the three dimensions of color, the hue is the most critical in terms of perceptibility and acceptability for normal color observers. The L C H color space identifies the hue as one of the three dimensions. A color is located using cylindrical coordinates with L being the same as in CIELAB and C and H computed from a and b. The coordinates of CIELCH (also see Fig. F5.1.11) are ... 

There is a striking similarity between the experimentally observed and the theoretically calculated profiles, and all four characteristic features occur in both. The calculated location of the minimum, which mainly depends on the vacancy mobility, is close to the location observed in the experiment. The computed temperature dependence of the depth of the minimum corresponds with the results of the measurement. Obviously, the stoichiometry polarization model of resistance degradation correctly predicts the conductivity variations. In particular the almost quantitative agreement of the very characteristic shape of the conductivity distribution proves the validity of the existing model described above. It should be noted that in the calculations only the hole mobility is chosen such that the theoretically and the experimentally observed depth of the minimum is similar, but all other parameters used in the simulation are taken from literature [77, 336, 338],... 

Kaminski, G. A., Friesner R. A. and Zhou R., A Computationally Inexpensive Modification of the Point Dipole Electrostatic Polarization Model for Molecular Simulations. J.Comput.Chem. (2003) 24 267-276. 

Elking, D., Darden T. and Woods R.J., Gaussian Induced Dipole Polarization Model. J. Comput. Chem. (2006) 28 1261-1274. 

The yield stress can in principle be predicted from the polarization model. Rigorous calculation of the movement of one sphere in a flowing ER fluid under an electric field requires computation of the dielectric and hydrodynamic forces on that sphere. But these forces depend on the location and movement of all surrounding spheres, which are themselves responding to similar forces. Thus, one must solve a many-body problem, and this requires computer simulation. 

The simplest method to correct for the inadequacy of the pairwise-additivity assumption is to use a classical polarization model" to allow for the induction of one or more dipoles in each molecule due to the electric field of the surrounding molecules. The details of such calculations are given elsewhere. Because of the multibody nature of such a calculation, and the need for iteration over all the molecules in the system, this adds a factor of 3-10 to the computational load of an MC simulation. 

Attempts to represent the three-body interactions for water in terms of an analytic function fitted to ab initio results date back to the work of dementi and Corongiu [191] and Niesar et al. [67]. These authors used about 200 three-body energies computed at the Hartree-Fock level and fitted them to parametrize a simple polarization model in which induced dipoles were generated on each molecule by the electrostatic field of other molecules. Thus, the induction effects were distorted in order to describe the exchange effects. The three-body potentials obtained in this way and their many-body polarization extensions have been used in simulations of liquid water. We know now that the two-body potentials used in that work were insufficiently accurate for a meaningful evaluation of the role of three-body effects. 

The experiment was based on a channel flow cell system (see Fig. II.6.2) with UVMs detection downstream of the working electrode (Fig. n.6.2d). Switching the electrode potential from the potential region with no faradaic current into a region with diffusion-limited faradaic current allowed the transient change in the UVAfls absorption to be monitored. The data analysis for this transient UVAds response was based on a computer simulation model, which allowed T>(TMPD) and T)(TMPD ) to be varied independently. Interestingly, the difference in Z)(TMPD) and T)(TMPD ) was relatively high in water and ethanol (15% slower diffusion of the radical cation) and considerably lower (5%) in the less polar solvent acetonitrile. This example demonstrates the ability of transient spectroelectrochemical experiments, in conjunction with computer simulation-based data analysis, to unravel even complex processes. 

It is evident that the computational results of the method critically depend on the quality of the parameterization and in particular on the point polarizabilities usually associated with atoms. The atomic polarizabilily parameters are generally obtained by fitting to either experimental or QM molecular polarizabilities or QM electrostatic potentials. The methods can also be divided into two groups additive and interactive models, depending on the level of interactions permitted between induced dipoles [20]. In the additive approach, polarizable sites are allowed to respond to an external electric field but not to permanent and induced multipoles on other sites within a molecule. In nonadditive, also called interactive, polarization models, instead, each of a molecule s polarizable sites is allowed to respond to an external electric field not only from other molecules but from other sites within the same molecule. Consequently, aU interacting sites polarize themselves. Under certain conditions, two inducible dipoles at short distances can cause a polarization... 

In Figure 2 the computed potential energy function for the KPC "simple model and the Polarization Model are displayed for the symmetric planar configuration shown. The fit is adeouate. Figure 3 shows the KPC potential versus the Polarization > odel potential in the reversed planar configuration indicated. The compromise nature of the "fit" is apparent. 

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