Magnetic properties finite nuclear

All of the above-mentioned investigations on the hydrogen atom were performed in the fixed-nuclei approximation, i.e. for the assumption of an infinitely heavy nucleus. In the absence of a magnetic field the center of mass (CM) and electronic motion separate exactly, and the influence of the finite nuclear mass on the behavior and properties of the atom can be taken into... 

Finally, we emphasize that the above potential F is inseparably connected with the finite nuclear mass. Assuming an infinite nuclear mass would simply yield F = 0 and the above-mentioned properties of the total potential f would disappear. In order to obtain the correct qualitative behavior and properties of the atom in a strong magnetic field it is, therefore, necessary to treat the atom as an inseparable two-body system. 

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. 

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