using first-order perturbation theory ,estimate the correction to the ground state energy of a hydrogenic atom due to the finite size of the nucleus, assume it's spherical nucleus.
you can employ the fact that the electrostatic potential fi
fi=Ze/R(3/2-r^/2R^ ,if r<R
=Ze/r ,if r>R
fi is elctrostatic potential
R is radius, chargeZe