  Entropic measures provide analytic tools to help us understand correlation in
quantum systems. In our previous work, we calculated linear entropy and von
Neumann entropy as entanglement measures for the ground state and lower lying
excited states in helium-like systems. In this work, we adopt another entropic
measure, Shannon entropy, to probe the nature of correlation effects. Besides
the results of the Shannon entropy in coordinate space for the singlet ground
states of helium-like systems including positronium negative ion, hydrogen
negative ion, helium atom, and lithium positive ion, we also show results for
systems with nucleus charge around the ionization threshold.
