Some high temperature superconductors require atmospheres of oxygen for the formation of the superconducting phase, and others do not. The former corrode more easily and includes the doped compound (La1.5Ba0.5)CuO4. In the latter class, Cu ions inhabit sites with large ionic potential, which apparently allows the formation of high charge states such as Cu (d8) or CuO+. An elementary code was used to examine potentials on a number of defect sites. Thus a high charge state on polyvalent Cu is a feature common to both types of superconductor. The use of Madelung potential calculations is reviewed for the construction of band energy diagrams, and for determining cohesion energies and stability of defects.