On the chemical potential/electronegativity equalization in density functional theory

General variational principles of the Kohn-Sham (KS) density functional theory are interpreted as the corresponding chemical potential/electronegativity equalization equations. The unconstrained (ground-state) and constrained (excited) electron configurations are examined for both the system global description and for the case of its partitioning into mutually closed subsystems, e.g., reactants. The chemical potential discontinuity for the integer numbers of electrons at zero temperature is stressed, and the KS orbital description of the charge transfer (CT) between reactants is discussed. Using the appropriate ensemble formulation of the KS theory the in situ chemical potential/electronegativity difference, the driving "force" behind the inter-reactant CT, is linked to the relevant KS frontier eigenvalues of polarized reactants.