where E and n are the DFT energy and number of electrons, respectively, and v is the external potential that uniquely defines the electron density of a system. The electronegativity scale defined by eq. ( ) was first proposed by Iczkowski and Margrave, and it implies Sanderson's equalization. Because the chemical potential is homogenous throughout a system at equilibrium, all atoms in such a system will, by the CDFT definition, have equal electronegativity. A well-known approximation to this electronegativity is the Mulliken scale, χ I , which can be obtained by a finite-difference approach. The Mulliken electronegativity can be expressed in DFT terms as: