The difference between ground states of different isotopes I have kind of a fundamental question. So whenever we talk about the ground state of an isotope, we usually denote that the energy is 0. However, that ground state is relative. For example, the ground state of 40Ca is not the same as the ground state of 44Ca. So, how does one calculate the numerical values of the difference between those ground states?
 A: Let's begin by most researched Hydrogen atom. Here is calculated absorption spectra of Hydrogen isotopes + Helium gas mixture at $9000 \,\text{K}$ temperature (compiled from here) :

So absorption spectra is different for Hydrogen-Helium, Deuterium-Helium and Tritium-Helium gas mixture. These spectra differences comes from differences of energy levels in various Hydrogen isotopes, including different ground-state energy of isotopes, which can be summarized as :
$$ E_1 = hcR\,\left(1+\frac{m}{M}\right)^{-1} $$
where $R$ is the Rydberg constant for an infinite-mass nucleus, $m$ is the mass of the electron, $M$ is the mass of the nucleus. 
So different number of neutrons in isotopes will change nucleus mass, which will result in different ground energy of isotope and consequently - it's different spectrum. Same spectrum difference between isotopes can be seen by comparing normal and "heavy" water ($D
_2O$) spectrum. Just in that case difference will come, because deuterium isotope will change water molecule vibrational energy. 
Thus, conclusion is that ground energy of isotope is never zero and depends on nucleus mass. The thing that you sometimes find ground energy defined as zero, can mean that energy of isotope levels is re-scaled or normalized by ground-energy level, i.e one can re-define energy level as $E_{n\to1} = E_1-E_n$. This would be relative energy of level, thus at ground level it can be zero now.
