In introducing the various CT metrics of Section 2 .2, it was not explicitly stated how the MOs {ψ i } and {ψ a } are to be defined, and we will consider several (seemingly reasonable) choices. The most obvious choice is to use the CMOs that diagonalize the Fock matrix and have well-defined one-electron energy levels. An opposite limit is to use MOs that are localized in space rather than energy. For that purpose, we will consider Boys-localized MOs, computed using the algorithm from Ref. 96. In Section 4 .1, we will consider Boys orbitals in some highly diffuse basis sets, where we were concerned whether small differences in the MO coefficients might lead to different localized orbitals in the iterative Boys procedure. For consistency, in all cases involving Boys orbitals, the SCF guess consists of a superposition of atomic densities generated in situ, using the target density functional and basis set.