(2.4.4) Xu : The AC path mentioned in contribution (2.4.2), which bridges the fictitious noninteracting Kohn-Sham system to the fully interacting real system, is one of the most important concepts in the development and understanding of DFT. The coupling-constant integration along the AC path defines the Kohn-Sham exchange-correlation functional, which also accounts for the kinetic energy of correlation. The more we know about the AC path, the better DFAs we can construct.