The first order correction to 0th order depends on size, vanishing for a point nucleus. However, it also depends on the electron charge density at the center of the nucleus. Since there are no electron charges in the nucleus, this term will be zero.
The Fermi contact contribution does not depend on size and since all the electrons stay outside the nucleus, this effect is non-existant.
Since Fermi contact contribution is 0, the Bohr-Weisskopf effect is the dominant correction, which does depend on size.
This topic was modified 3 years, 8 months ago by Ines Almeida.