(2.5.2) Kronik : Regarding contribution (2.5.1): Interestingly, decades after his seminal DFT work, Walter Kohn formalized the idea of almost-additive local contributions by introducing the concept of "nearsightedness" of electrons in many-atom systems; see also contribution (5.4.7). Nearsightedness means that (with some caveats), for a fixed chemical potential, local electronic properties, such as the density, ρ(r), depend significantly on the effective external potential only at nearby points. (2.5.3) Ayers, Chattaraj, Chermette, De Proft, Fuentealba, Geerlings, Liu, Vela, and Yang : In the variational equation of DFT, the Lagrange multiplier µ was identified by Parr and coworkers in 1978 as the partial derivative of the energy E with respect to the number of electrons N, at constant external potential v(r),