Potential-Derived Monopole Models with Additional Nonatomic Sites

POTENTIAL-DERIVED MONOPOLE MODELS WITH ADDITIONAL NONATOMIC SITES 

There are situations where a net atomic charge model does not give the desired accuracy of fit to the electric potential. Since the least-squares fitting procedure is a curve-fitting process, it is expected that addition of new variables of appropriate mathematical form will improve the fit. The addition of a new fixed site adds implicit variables, x, y, z, of the site location and an explicit variable, q, the site charge. The new site can be treated as a dummy atom with net charge q, and this charge can be optimized. 

The optimum location of additional charge sites present a more challenging question. Of course, one can simply look for a location where the fit to the electric potential is best. However, this optimum site location and its charge may not make sense in terms of chemical intuition or experience. Chemists traditionally seek models for molecular properties that are transferable between related molecules. So we are faced with the problem of finding site locations that are consistent with chemical intuition and yield the best transferability properties. If the model is transferable, it provides a basis for intelligent discussion or understanding of properties of related molecules. 

The variation of this approach merges with the PD point multipole models discussed in the next section. It is well known that an atom in a molecule can possibly have a dipole, quadrupole, etc. An atomic dipole, quadrupole, etc. can be simulated by a distributed multipole. For example, an atomic dipole ascribed to a lone pair can be simulated by placing a lone pair charge site near the atom, at the appropriate direction and distance. An atomic quadrupole can be simulated by placing several monopolic sites near the atom. 

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Potential-Derived Monopole Models with Additional Nonatomic Sites 249... 

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