Charge description

Figure 4. Two spherical ions, with equivalent charges at different centers. In a purely center of charge description the two ions are equivalent, but their interactions will be governed by a different balance of dispersive and Coulombic interactions.

The process of bond formation is poorly modeled with a point-charge description that contains the vital information about the relative valence states of dissimilar atoms in terms of ionization radii only. The predicted course of reaction is outlined in figure 4. Atomic promotion is once more modeled [136]... 

FD methods are widely used in combination with classical point charge description of the solvent (FEM is less used see You and Harvey, 1993, for a recent implementation of this approach). The first formulation of the method is due to Warwicker and Watson (1982) (see Davis and McCammon (1990) for a review). The use of 3D numerical grids makes the calculations generally computer intensive, and, in addition, the evaluation of AGei is rather inaccurate. 

The surface charge description of amphoteric mineral oxides has been given considerable attention in literature [see e.g. 22, 28-30], but, to some extent, it is still controversial. The controversy essentially boils down to the representation of the surface group(s) that act as proton adsorption site(s). In the classical [22, 30] and most common approach the surface is assumed to be monofunctional with a postulated surface oxygen group that can undergo two protonation steps, each governed by its own pKn value ("two-pKn model) ... 

With a view to checking whether or not the results of the electrostatic calculations performed on Wa-h2o section with constant R are able to provide a sufficiently reliable picture, we shall compare a few sections obtained by the approximate method with some relying upon the usual ab initio SCF calculation (in terms of a minimal basis set). In the electrostatic calculations, since one of the partners is always an H20 molecule, the point charge description of paragraph VIII. B may be utilized. The interaction energy W will consequently be obtained by applying Eq. (9) ... 

Other metai centers Validity of the formal charge descriptions... 

The selection of a suitable force field for crystal calculations can also be based on the root mean square (RMS) deviation of the atomic positions used to characterize the deviation of the energetically minimized crystal structure from the initial structure, which is determined through the use of x-ray diffraction data. The smaller the RMS value, the more the minimized structure resembles the experimental structure, and hence the more accurate the atomic charge description and governing energy expression are considered (Givand et al. 1998). 

MEAN FIELD ELECTROSTATICS BEYOND THE POINT CHARGE DESCRIPTION... 

This chapter explores the number of mean field constructions for ions whose structure goes beyond the point charge description, the representation used in the standard Poisson-Boltzmann (PB) equation. The structural details omitted within a point charge picture can be related to electrostatic structure of an ion,... 

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