Mean atom charge

The presence of the q B term with its implied distance dependency means that the charges depend upon the molecular geometry. Thus, should the conformation of a molecule change the atomic charges will also change. Just three parameters are required for each atom in the system (the electronegativity, the idempotential and the covalent radius). 

Although the localization by energy criteria (Edmiston-Ruedenberg) may be considered more fundamental than one based on distance (Boys) or atomic charge (Pipek-Mezey), the difference in computational effort means that the Boys or Pipek-Mezey procedures are often used in practice, especially since there is normally little difference in the shape of the final LMOs. 

Integration of these component peaks, with appropriate corrections applied for different photoionization cross-sections and inelastic mean free paths, gives the electron populations listed in Table 4. The atomic charges obtained are consistent... 

The charge associated with a given atom is an often used and important concept, but one that has physical meaning only when defined for a bounded, open system. The atomic charge for an atom A, denoted by q(A), is defined as q(A) = ZA-N(A), the dif-... 

In Eqs. (16) and (17), a and P refer to atoms of the solute and solvent, respectively q is the permittivity of free space, Qa and Qp are atomic charges, and Rap is the distance between atoms a and p. The parameters eap, aap, Aap, Bap and Cap can either be assigned by fitting to experimental data or can be the arithmetic or geometric means of literature values for the individual atom types.10,65,66 The atomic charges are commonly determined by requiring that they reproduce the calculated molecular electrostatic potentials.10 In order to provide better descriptions of the solvent s structure, Eqs. (16) and (17) are generally extended to include solvent-solvent intermolecular interactions. 

The potassium salt of the 2,2 -dipyridyl acetylene anion-radical represents another important example. In this case, the spin and charge are localized in the framework of N-C-C=C-C-N fragment. The atomic charge on each nitrogen atom is -0.447, that is, close to unity in total. The energy of this ion pair is minimal when the potassium counterion is located midway between the two rather close nitrogen lone pairs. Such a structure is consistent with the fact that the ESR spectrum of this species is almost insensitive to temperature. It means that the counterion does not hop between two remote sites of the anion-radical (Choua et al. 1999). 

At a quite different level of approximation, this class of compounds was investigated by means of STO-3G calculations involving a detailed optimization of all the geometric and exponent parameters [42]. The Mulhken net atomic charges and the... 

The (at least approximate) validity of Eqs. (6.12)-(6.14) is clearly demonstrated by the remarkable quality of intrinsic CN bond energies and of bond dissociation energies calculated by means of nitrogen net atomic charges deduced in this manner. 

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