A different strategy was proposed in Ref. 229 (see Fig. ). There, a combined criterion of orbital kinetic energies and orbitalwise atomic charges was employed to directly determine active orbitals in PbE calculations as those orbitals that significantly change during a reaction. This can be regarded as a more direct active-system definition in the context of PbE, since the projector requires a partitioning of the occupied orbital space, rather than a partitioning on the basis of "atoms" or "densities". This was later refined using an orbital alignment criterion , transferred to WF-in-DFT embedding , and to multilevel coupled-cluster hybrid methods . The direct orbital selection from Ref. 229 also automatically solves the problem of selecting subsystems in an "even-handed" manner, which was brought up in Ref. 236.