Static point charges

Consider carefully the implication that the so-called magnitude of the field at a point has been defined as the magnitude of the result of the interaction of the field with a unit static point charge. In other words, the effect of the field has been erroneously defined as the causative field itself, a nonsequitur. The effect cannot be the cause. 

The PPD and shell models are nearly equivalent in this sense, because they model the electrostatic potential via static point charges and polarizable dipoles (of either zero or very small extent). Accuracy can be improved by extending the expansion to include polarizable quadrupoles or higher order terms.The added computational expense and difficulty in parameterizing these higher order methods has prevented them from being used widely. The accuracy of the ESP for dipole-based methods can also be improved by adding off-atom dipolar sites. 

The VSEPR model is based upon Lewis structures which assume that all valence electrons are paired and a chemical bond requires two electrons. The model is limited to simple compounds of the main group elements (s- and p-blocks) and some transition metal ions (those with d° and d configurations). It can only predict exact bond angles for molecules with no lone pairs. It is theoretically unsatisfactory since electrons do not behave as static point charges and it provides no information about the stability of molecules. However, VSEPR theory is a simple and powerful model that satisfactorily predicts and explains the shapes of a large number of molecules and ions from the s- and p-blocks. 

Analysis of ion atmospheres around highly charged macromolecules has traditionally been performed using numerical solutions to the nonlinear Poisson-Boltzmann (P-B) equation (Anderson and Record, 1980 Bai et al, 2007 Baker, 2004), in which the macromolecule is approximated as a collection of point charges embedded in a low dielectric cavity surrounded by a high-dielectric solvent. This approach utilizes the precise three-dimensional structure of the macromolecule (albeit in a static sense). We would not expect such a framework to capture subtleties, which are dependent on the partial dehydration of ions. 

Static electrical charges in the machinery and stock can be important in attracting contamination, and facilities for electrical discharge at appropriate points also will be needed. 

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