Point nucleus magnetic properties

It must first be remarked that there is no way to compare these computed NICS values with an experimental measurement, since there is no nucleus (typically) at the center of aromatic rings. The values in Table 3.24 demonstrate some small basis set dependence. Schleyer recommended the use of diffuse functions for the evaluation of NICS. NICS is a local measure, a magnetic property at a single point. There are concerns over using such a local property to evaluate the global nature of... 

Thus far in our discussion of relativistic expressions for properties we have assumed that the nuclei are represented by point charges. However, schemes for actual calculation of relativistic wave functions normally use nuclei with finite size in order to avoid problems with the weak singularity of the Dirac equation at the nucleus—and also because the nucleus really does have a finite size. The use of a point nucleus to calculate properties therefore appears somewhat inconsistent. At the very least we should know what errors we incur by using a point nucleus, and we will therefore discuss the low-order effects of finite nuclear size for electric and magnetic fields. 

Finally, in Sect. 6, we have briefly given some examples for physical properties or effects, which involve the nuclear charge density distribution or the nucleon distribution in a more direct way, such that the change from a point-like to an extended nucleus is not unimportant. These include the electron-nucleus Darwin term, QED effects like vacuum polarization, and parity non-conservation due to neutral weak interaction. Hyperfine interaction, i.e., the interaction between higher nuclear electric (and magnetic)... 

The property of a nucleus that allows magnetic interactions, i.e. the property possessed by and but not by is spin, if you conceive of a and nucleus spinning, you can see how the nucleus can point in one direction—it is the axis of the spin that is aligned with or against the field. 

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