4 In the Hartree theory for multi-electron atoms, electrons are assumed to experience an effective potential energy, U(r), which depends on their radial coordinate, r. Give expressions for U(r) that would be appropriate for the ground configuration of twice ionized calcium (i.e. Ca +) for the following two limits: (a) r is very small (r - 0) (b) r is large (r - co). [4] [Calcium has atomic number Z = 20.]