The rms radii for protons and neutrons are defined as ( or )

The charge radius is obtained from the proton radius by taking into account the proton charge distribution in an approximate way, i.e.,

with fm. The mean gaps are the average values of the pairing fields

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The fluctuations of the particle numbers are defined as and given by .

Finally, the
rearrangement energy, which comes from the density dependence of
the force, and which shows how much the force is modified by the medium
effects, is given by

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