(2.4.2) Perdew and Savin : In many cases, the energy and wave function of the interacting system can be connected smoothly to those of the Kohn-Sham noninteracting system of the same electron density. Then the exact exchange-correlation energy for that density becomes an integral over the strength of the electron-electron interaction, which subsumes both the potential energy of exchange and correlation and the kinetic energy of electron correlation. The adiabatic connection (AC) and the idea of modelling the pair density associated with it 100,119-121 served as the key inspiration not only for passing from LDA to GGAs , but also for making the step to hybrid functionals 124 . Note that it is not necessary to use the pair density in the adiabatic coupling; one can use the first-order density matrix as well -see, for example, ref. 125.