  Popular approximations to the exchange-correlation (xc) energy of density
functional theory do not yield the spatial `step' structures in the exact xc
potential which are necessary to describe dissociation and electron excitation
with the Kohn-Sham (KS) system. Via the discontinuity in the derivative of the
xc energy as a function of electron number I derive exact analytic expressions
in terms of the KS single-particle energies for the height of the step in the
xc potential between a variety of open- and closed-shell atoms within stretched
molecules.
