Steric effects have been quantified in DFT by the Weizsäcker kinetic energy functional. Its functional derivative has been employed to predict stereoselectivity. Pauli energy was validated as a robust identifier for double, triple, quadruple, and even higher covalent bonds. 197 The electron localization function (ELF) The concept of the adiabatic connection (AC), conceived within the DFT framework and successfully used to develop approximations to exchange-correlation functionals -see contributions (2.4.2)-(2.4.7) -has inspired the development of methods for calculating the dynamical correlation energy in wave-function theories. In the general AC theory developed in ref. 205 and elsewhere, one is not restricted to adopting a noninteracting Kohn-Sham system as a reference system, corresponding to a vanishing coupling constant. If, instead, the reference wave function consists of a combination of Slater determinants and orbitals are partitioned into noninteracting groups (most commonly into doubly occupied (inactive), fractionally occupied (active), and unoccupied (virtual) orbitals, as in multiconfigurational self-consistent-field (MCSCF) theory) then, by following the AC path, the limit of no correlation is smoothly connected with the full electron-correlation limit.