All of the adiabatic pathways above focus on the density-fixed case, relevant to Kohn-Sham DFT. However, if one notes the conjugate relationship between F[ρ] and E[v], a natural alternative is a potential-fixed AC, a possibility that has also been explored numerically. Since the density is no longer fixed, the calculations of the AC pathway are in the potential-fixed case much simpler to perform, but the noninteracting reference system (the bare nucleus system) is further from a realistic electronic system than its Kohn-Sham counterpart. Recently, other ACs have been developed that do not insist on a fixed density along the adiabatic pathway -see, for example, ref. 162 for an AC that recovers the Møller-Plesset series as its low coupling-strength expansion. The utility of the AC as a concept for understanding new theories based on these alternative pathways and their relative pros and cons compared with the Kohn-Sham approach underlines its importance as a concept in electronic-structure method development.