Force related atomic charges

Dispersion or London force related to the nonpolar properties of the van der Waals attraction exists between all adjacent pairs of atoms or molecules. As mentioned above, its origin is the instantaneous charge redistribution in one molecule polarizing the electron clouds in adjacent molecules, resulting in nonpolar (or nonspecific) intermolecular attraction. 

There are two factors that are closely associated with the structure and behavior of atoms. The first of these is known as effective nuclear charge. The nuclear charge is related to the number of charged particles (protons) in the nucleus. As the nuclear charge increases, there is an increase in the attractive force between the nucleus and the electrons. Nuclear charge increases from left to right across a period. 

A problem related to obtaining MM potentials for reactions is to devise functions for the excited states of molecules. Blair et al. [74] and DeBolt et al. [75] have parameterized MM potential energy functions for the first excited states of formaldehyde and acetone respectively. The force fields in both cases were of the same form as for the ground states, except that the parameters, especially the partial atomic charges, were altered to reflect the changes that occur in the excited state. 

To conclude this section we note that the atomic charges are defined in this analysis by the forces on the nuclei rather than by fitting the electrostatic potential of the molecule. AccJ)rdingly, these charges are termed force related (FR) atoimc charges. ... 

An analysis of the atomic quadrupolar tensors similar to that described a ve for the APT and the FR atomic charges yields the following definition for the force related atomic dipoles ... 

The Hamiltonian considered above, which connmites with E, involves the electromagnetic forces between the nuclei and electrons. However, there is another force between particles, the weak interaction force, that is not invariant to inversion. The weak charged current mteraction force is responsible for the beta decay of nuclei, and the related weak neutral current interaction force has an effect in atomic and molecular systems. If we include this force between the nuclei and electrons in the molecular Hamiltonian (as we should because of electroweak unification) then the Hamiltonian will not conuuiite with , and states of opposite parity will be mixed. However, the effect of the weak neutral current interaction force is mcredibly small (and it is a very short range force), although its effect has been detected in extremely precise experiments on atoms (see, for... 

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